Do not capitalize inline code, markdown style

[cuihtlauac]

Do not capitalize stuff surrounded by backticks:

echo 'The general synopsis of the `grep` command line' | titlecase
The General Synopsis of the `grep` Command Line

[wezm]

I can see why this would be useful. However, currently title case has no knowledge of Markdown, it's just handling plain text. Perhaps there should be a markdown mode to handle this and #13?

[cuihtlauac]

Yes, a markdown mode makes sense.