Split collectors, add aggregate collector, rename xhgui

Wasn't sure how to title this set of changes, but I will describe them:

This PR renames XHGui to XHProf.io which is more inline with the documentation. XHGui is another, separate project that used to share its name - this has been clarified. The default XHProf implementation was added. An aggregate collector is present which is the default collector for requests and console commands which supports collecting to many child collectors. A compiler pass controls which collectors are tagged with data_collector, by default disabling unused collectors from polluting the container which might only be useful in some scenarios, but was for me because I'm not always using them in every environment. Lean containers just feel nice :)

I think that's it! Critique is 110% welcome, but I can't promise I'll have time to implement any suggestions, although I will do my best.