Adding a force flag to options to not exit on errors found.

[belshire]

Adding a force flag to not fail the task but still report when errors and warnings are found. Inspiration came from the implementation in the jshint grunt task

[nschonni]

Probably good to add an example in the README because I think this is a common scenario due to a few outstanding bugs in parser-lib (IE CSS expressions, @document support, etc...)

[belshire]

Added documentation on usage for the force flag to the README and the individual .md files in the docs folder.

[nschonni]

@shama any objections to this?

[shama]

I spoke with @cowboy today in IRC about whether tasks should fail or warn on error. He said warn on errors. So pretty soon I'll go through the tasks and make that consistent (as I know some do fail on errors and some don't).

In which case we shouldn't need a force option implemented here or within the other tasks anymore.

/cc @tkellen @sindresorhus @jsoverson

I appreciate you putting together this PR though @belshire! and thanks for reviewing @nschonni!