Add a safe `exec` function

[gsingh93]

I'm kind of surprised this doesn't already exist in the PHP world, but we should have a safe and easy to use implementation of exec. Facebook has a good implementation internally that can be used like execx('echo %s', 'foo;rm -rf'), and this will echo the argument safely. It's probably not a good idea to pull out FB internal code like that though, and it has more features than we actually need.

This seems similar, maybe we can pull out this code: https://secure.phabricator.com/book/libphutil/function/execx/

[fredemmott]

If/when you do this, please take advantage of \HH\FormatString<T> so the typechecker understands the patterns and verifies the appropriate arguments are passed in.