Add option to not highlight trailing spaces.

Add an option not to highlight trailing spaces as errors for hamlet
files.

Also add documentation to the README about the two possible options that
the user can set.
Fix comments from pull request.

- Added blank line in README.md and add "vim" syntax highlighting so
  that the example options show up highlighted.

- Removed unnecessary comment from syntax/hamlet.vim and fixed some code
  formatting.
Merge pull request #20 from cdepillabout/add-option-to-not-highlight-trailing-space

Add option to not highlight trailing spaces.