chore: Use grep -E / grep -F instead of egrep / fgrep (#2164)

Ensures that the -E or -F option, when used, is the first option
* i.e. grep -oE => grep -E -o

Updates _bash-it-grep to invoke grep with just the provided arguments
* This function was (and still is) unused, but decided this new functionality was actually more useful

Introduces _bash-it-fgrep to invoke grep -F

Removes type -P egrep from the _bash-it-*grep functions

For usages that were already going to be modified, use -F if appropriate
* Does not touch grep usages that may have benefited from -F, but were not otherwise considered for this PR

Adds shellcheck header to modified .bash files that didn't already have it

themes/rjorgenson/rjorgenson.theme.bash

@@ -1,3 +1,5 @@
+# shellcheck shell=bash
+
 # port of zork theme
 
 # set colors for use throughout the prompt
@@ -50,7 +52,7 @@ function is_integer() { # helper function for todo-txt-count
 
 todo_txt_count() {
     if `hash todo.sh 2>&-`; then # is todo.sh installed
-        count=`todo.sh ls | egrep "TODO: [0-9]+ of ([0-9]+) tasks shown" | awk '{ print $4 }'`
+        count=`todo.sh ls | grep -E "TODO: [0-9]+ of ([0-9]+) tasks shown" | awk '{ print $4 }'`
         if is_integer $count; then # did we get a sane answer back
             echo "${BRACKET_COLOR}[${STRING_COLOR}T:$count${BRACKET_COLOR}]$normal"
         fi