gfm-todotxt highlighting

Add highlighting for embedded TODO text within Github Markdown files. Adds due date highlighting to imdone-atom tasks.

TODO

• #todo:30 Highlight dates when board initializes
• #todo:10 Add video to readme
• #todo:20 tests
• #todo:60 add settings for different due date durations
• #todo:70 add settings for different color for each due date duration

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Examples with due dates:

• #todo:0 do something due:2016-01-01
• #todo:50 do something else due:2016-02-03