support config file

[epicallan]

A config file may be added to a repo that a bot has access to. This config file can then be used to change bot behavior.

Possible config values may include;

• keywords to listen for i.e TODO, FIXME, BUG
• boolean flag for auto-deleting checked out or no longer existing TODO items in source code
• issue labels to add to auto created bots
• mode in which bot should run i.e add issues on PR or on pushing to a specific branch (default master) see https://github.com/freelyformd/github-todos/issues/65
• files to ignore (regex pattern)
• root directories of the project such that it is easy to traverse project for comments on bot install see https://github.com/freelyformd/github-todos/issues/54 defaults can be src, app etc