Add ability to limit to current function

[notbaab]

A little unsure how to do this. It will definitely have different code paths depending on the language features.

For C++ like languages, using the scope may still work, we would do something similar to expand to scope command.

For whitespace significant languages, we could just use the indentation level? That is a future idea anyway.

For Ruby or Bash languages, i.e. where expanding to scope pretty much expands it to the whole file, it needs to do something similar to BracketHighlighter and have a regex of keywords that define a function block.

It would be possible to include BracketHighlighter as a dependency, and that would actually help out the other above issues as well, there a tons of helper functions for these types of cases, but it adds a ton of functionality that a user may not want, and also turns this tiny package into a much larger beast.