Added possibility to change the download URL

[ynx-zeissl]

In order to cache the mockingbird package on a local server and downloading it from there, the hardcoded github link must be changeable.
I havn't tried this MR locally so please review it.
Link to my already created Feature Request: #200

[andrewchang-bird]

Thanks for the PR! I’ve made a few changes and verified that it works locally:

% make install
% mockingbird download starter-pack 
Downloading asset bundle from https://github.com/birdrides/mockingbird/releases/download/0.16.0/MockingbirdSupport.zip
Temporary asset bundle data stored at file:///var/folders/27/f9qsltsn4dd53snd2tpr9rkh0000gn/T/CFNetworkDownload_GQSSnj.tmp
Extracting downloaded asset bundle to /Users/typealias/mockingbird
Successfully loaded asset bundle 'starter-pack' into /Users/typealias/mockingbird
% mockingbird download starter-pack --url https://github.com/birdrides/mockingbird/releases/download
Downloading asset bundle from https://github.com/birdrides/mockingbird/releases/download/0.16.0/MockingbirdSupport.zip
Temporary asset bundle data stored at file:///var/folders/27/f9qsltsn4dd53snd2tpr9rkh0000gn/T/CFNetworkDownload_SrJYeC.tmp
Extracting downloaded asset bundle to /Users/typealias/mockingbird
Successfully loaded asset bundle 'starter-pack' into /Users/typealias/mockingbird

[ynx-zeissl]

Thanks :)