Updating include to account for single groups (backward compat)

A breaking change was introduced in the latest change which changes the way custom regular expressions work.

Previously, a regex following the form, for example,  `/^\/\/\+\s*include\s+['"]?([^'"]+)['"]?\s*$/`, would work without issue.  The breaking change was the introduction of another group to the regex for indentation first, meaning that the file name (`match[2]`) becomes undefined unless this regex is modified to include another group somewhere.

This pull addresses that by defaulting to `match[1]` being the indent and `match[2]` being the file location, but if file location is undefined it'll use the first group instead.  This preserves the new functionality of indentation and is backwards compatible.