The -l flag to the replace command means "literal" and will treat
the search term literally instead of as a regular expression.
The command bar also now supports expanding environment variables
and running expressions through the shell and using the result
in the command.
This commit also switches from yosuke-furukawa/json5 to flynn/json5
because yosuke-furukawa/json5 does not provide a license.
Ref zyedidia/json5#1
Closes#775
