added bin directory for misc tools

Added `bin/sgit`.

This directory is NOT intended to become a huge software project
right now.  For now it is just a convenient place to store tools.
Please avoid overthinking, over documenting tools under bin.

Later on there will be a tool that builds links in the web site
from JSON files, perhaps forms HTML files from markdown files, etc.
However we will have to make major progress on issue #1, issue #2,
issue #3 and issue #4 before such code can be written.

In some cases tools put under bin might go away later on
if they are no longer needed.

bin/sgit

@@ -0,0 +1,72 @@
+#!/usr/bin/env bash
+#
+# sgit - sed on all files under git control
+#
+# usage: sgit <sed command...> <glob>
+#
+# sed must be GNU sed and we use -i'' just in case it's not. Even so sed under
+# macOS will fail. This script could be improved to check for GNU sed and it
+# might be an easy way to fix it in other sed implementations but it works for
+# my purposes and since I want to work on the reason for this script (and not
+# the script itself) I'm not going to worry about that.
+#
+# DISCLAIMER AND WARNING: this is hack and most likely error prone!
+#
+# - Cody Boone Ferguson (@xexyl)
+#
+
+# firewall
+#
+# first check that this is a git repo!
+
+git status 2>/dev/null 1>&2
+status="$?"
+if [[ "$status" -ne 0 ]]; then
+    echo "$(basename "$0"): ERROR: ${PWD} not a git repository" 1>&2
+    exit 1
+fi
+
+# check number of args
+if [[ "$#" -lt 2 ]]; then
+    echo "$(basename "$0"): ERROR: usage: $(basename "$0") <sed command...> <glob>" 1>&2
+    exit 2
+fi
+
+export SED_COMMANDS=""
+export GLOB=""
+# go through the command line, extracting the sed commands. When we've reached
+# the end (one arg left) we know we have the glob to pass to git ls-files.
+# We do it this way because apparently $BASH_ARG[VC] are not set unless in
+# extended debugging mode (shopt -s extdebug).
+i=0;
+while [[ "$i" -le "$#" ]]; do
+    SED_COMMANDS="$SED_COMMANDS -e $1";
+    ((i++))
+    shift 1
+done
+GLOB="$1"
+
+# This is not the best performance because it does this on all files of the glob
+# rather than only those files matched by the glob with matching text but it's
+# better than it used to be (it used to run git ls-files on the repo for each
+# sed command so the more you had the slow it would become as it had to get the
+# list of files each time!).
+#
+# A better approach would be to somehow use git grep to find files that match
+# but this would complicate the command line (at least in anyway I can think of
+# how to do it) so we do it on all files in the repo.
+#
+# But these would possibly require additional args to the command and
+# unnecessarily complicate matters. Anyway it's a hack so it doesn't have to be
+# perfect (not that there even is such a thing as perfect).
+#
+# NOTE: it is a burden on the user to specify a glob that will only match
+# regular files. Although we could try and figure out if each matching file is a
+# regular file it would mean we would have to go through the list a line at a
+# time and then add it to a new list if it's a regular file and this seems
+# unnecessary. Besides doing '.' will not cause a problem so it might be fine.
+
+# shellcheck disable=SC2086
+# SC2086 (info): Double quote to prevent globbing and word splitting. We can't
+# disable this because we need to have word splitting.
+git ls-files "$GLOB" | xargs sed -i'' $SED_COMMANDS