sed command wasn't properly removing color escapes

Previously, sack wasn't correctly sanitizing the colored results from
ack or ag. This patch changes the sed command used to sanitize the
colored results from using the hex representation of an escape, \x1B, to
using an escape literal, . This corrects the problem.
Enable shortcut command to use $EDITOR

`create_shortcut_cmd()` now checks to see if `$EDITOR` is set, and if so
uses that value instead of `$sack__default_editor`.  Also, I used a
heredoc rather than repeated echo statements to populate the shortcut
command because it made editing the command's content much easier.
Merge branch 'EDITOR_support' of github.com:hakamadare/sack