Added support for LaTeX files

[cfgnunes]

Added support for LaTeX files, in which the comments start with the symbol %.

[aljoshakoecher]

I just came across the issue that todo-tree doesn't pick up to-dos inside LaTex comments. Awesome to see that there is already a PR 👍
Hint for anyone who might be in the strange situation that requires adding to-dos right now and cannot wait for this PR: Just prepend your to-do with something that todo-tree picks up e.g. //:

Some Text...
% //TODO: A To-Do that will get picked up
Some more text

[Gruntfuggly]

Hi, sorry to disappoint you, but this won't get merged. I no longer update the default regex as it causes too many issues for people who use the original default. Every time I've done it (after convincing myself it wouldn't have any effect) I've had to back it out again.

The default is really only intended to be a starting point - it would be impossible for the default to work for everybody.

The wiki contains some example configurations for LaTeX:
https://github.com/Gruntfuggly/todo-tree/wiki/Configuration-Examples