Addition of an example 'custom' Flask Blueprint for experimentation

[RichardBruskiewich]

@jrobinso my interest in this project today was simply to facilitate exploration and tinkering with igv.js data integration on my local machine. Along the way, I have sent a few tweaks to the project. However, a significant tweak still resides on my local machine. I'd love to issue a pull request for you to review; however, the project seems slightly too restrictive to do this at the moment.

That said, let me explain what I have: I have simply created a new "custom.py" Flask Blueprint alongside the ucsc and alignments blueprints, which simply allow access to (slightly tweaked) custom-track-click, custom-popover and customReference html templates, as examples inside this Flask project. Maybe, though, this idea could be generalized somehow to connect this project to the main igv.js. Maybe embedding the primary igv.js repo as a submodule might work(?).

[jrobinso]

@RichardBruskiewich I'm neither a flask nor python expert. I'm happy to entertain pull requests but will rely on other team members here to provide input on python. I'm not sure which project you are referring with wrt "too restrictive", or what that means.

[RichardBruskiewich]

Github commit/pull request restrictions. Actually, I'm being a bit slow here: I should just 'fork' your project then issue pull requests from there, ok? I've just gotten lazy on this simply due to my engagement in some large projects using Github within which I have some organisational membership.