Extensible definition dictionary

To avoid polluting our main formula dictionary (i.e. `PredefinedTexFormulas.xml`) with various localized formula names (like [this](https://github.com/ForNeVeR/wpf-math/pull/44#issuecomment-282280393)), I propose to add a dictionary extension mechanism.

Users should be able to provide theirs own dictionary files that will be merged with the existing definitions.

Also, these Russian localized definitions should be extracted from the default dictionary and should became opt-in functionality _(I still want to redistribute them alongside WPF-Math because personally I need those, and I think they're generally useful for people around the world)_.

And don't forget to add end-user documentation explaining how to make and apply these definition files!