Added support for custom fonts

[gtrafford]

I added support for custom fonts. I created an issue about this (#109) but there has been no reply.

[VisualMelon]

Sorry you've not had any response. I don't know if there is anyone in the team at the moment who is able to test the Xamarain components apart from Xamarin.Forms.

The azure devops build doesn't seem to touch the android stuff; the appyevor failure seems to be a version mismatch (pretty sure the code compiled fine). Could you stuff the lookup logic into its own method?

I'm rather busy at the moment (and have a long list of OxyPlot things I need to get done as well), but I'll see if I can borrow some devices with which to test.

[janusw]

Very late review: It seems you're only touching the Android implementation, so presumably your solution will only work on Android?

In general it's a nice idea to support custom fonts, but we should make sure that it works on all platforms (most importantly Android and iOS, but also Mac and UWP).