Custom Image API

[tneotia]

Fixes #130, #153, #172, #301, #386.
Implements #146, #156, #183, #235, #278, #496.
Todo #162, #304, #478.

Discussion at #497

See example/main.dart and html_parser.dart to see how the implementation works.
For those who are wishing for headers to be passed to images, please test this out against your use case and comment if you come across any issues.

[tneotia]

@ryan-berger let me know if this is something you'd like to merge after you take a look at it - I plan to extend the SVG functionality to support loading from assets and from base64, and also support pre-caching them, so I can either include all that in this PR (if it would be merged) or make a different one just for this.

[YeFei572]

good job!

[ryan-berger]

Are we going to close this one due to your other proposal and @erickok 's proposal @tneotia ?

[tneotia]

Are we going to close this one due to your other proposal and @erickok 's proposal @tneotia ?

Yep, I would just leave it be until we get either of those merged and then we can close this one and the other proposal that wasn't accepted.

[tneotia]

Closing since we are going with eric's PR (this is the old version anyways)