Adds monospaced font to expression input

Trying to read regular expressions with variable width fonts can
get a bit hard to read. This patch changes the expression input and  5
options input to `menlo`, falling back to `courier`, and finally to
`sans-serif`.

Screenshot: http://cl.ly/image/1b0M1F3x470Y

adambrodzinski@gmail.com
Merge pull request #18 from AdamBrodzinski/mono-spaced

Adds monospaced font to expression input