Custom command completion

[slideclimb]

Fixes #1359

Summary of additions and changes

• Fix the completion inside parameters of custom commands.
• Fix the insertion of a custom command, now also inserts {} for each required parameter.

How to test this pull request

\newcommand{\hello}[1]{hello #1}

\hello{\te<caret>}  % Auto completion list shows
\hell<caret>  % Use auto completion to insert \hello{<caret>}