Allow to use custom request matcher

[ihmels]

This adds an option to allow the use of a custom request matcher, e.g. to exclude certain paths.

This fixes #206 partially.

[tooltonix]

I need this feature urgently at the moment, please merge.

[Seldaek]

This would need to be added to the docs still, and ideally it should be made clear that using this renders the hosts option useless/unused.

[ihmels]

I added the request_matcher option to the docs. While writing this, I noticed that it also makes sense to ignore the content_types option when a custom RequestMatcher has been configured.

[ihmels]

@Seldaek, can you take another look at this?