Add the ability to namespace vue2-filters

[kball]

This gives a way to fix/work around issue #52 and #53.

There may be better approaches. I chose this one because it leaves the default behavior unchanged and implements a basic way to do namespacing, but let me know if you prefer a different approach.

[freearhey]

Thank you for your PR, but I decided to just give up the global mixin registration. Thus, those who need these methods can simply register Vue2Filters.mixin locally.