Theming markdown

Cleaning up the markdown view a little bit. Tasks with markdown bodies are nicer to read, now.

Test Results:
```
ruby-docket :) (theming_markdown)
$ bundle exec rake test
Run options: --seed 49102

# Running tests:

............S..........................................................

Finished tests in 7.348384s, 9.6620 tests/s, 25.4478 assertions/s.

71 tests, 187 assertions, 0 failures, 0 errors, 1 skips
Coverage report generated for Unknown Test Framework to /Users/kristofer.white/Code/ruby-docket/coverage. 267 / 283 LOC (94.35%) covered.
```