Allow escaping of formatting sequences with a backslash

Summary:
Say "\__init__.php", unless you actually want the output to be
"<b>init</b>.php".

Test Plan:
Ran "arc help liberate", and tested with:

// not underlined
echo(phutil_console_format("\__init__.php\n"));
echo(phutil_console_format("lol \__init__.php\n"));

// underlined
echo(phutil_console_format("__init__.php\n"));
echo(phutil_console_format("lol __init__.php\n"));

Reviewed By: epriestley
Reviewers: epriestley
CC: aran, epriestley, edward
Differential Revision: 766