Support ` for delimiting monospaced text in Remarkup

Summary:
Command line commands are often denoted by ` in plain text. It is natural to convert them to monospaced text in formatted text.
Other tools (such as GitHub flavored Markdown) use the same delimiter.
Delimiter ## is problematic in commit messages because git eats all lines starting by #. So it is not possible to start a line with monospaced text.

I am not aware about any conflits, old syntax is still supported.

Test Plan:

`test`
`test`s
`test
test`

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1773