add text/html writemime for MethodList and Method (fix #4952)

[stevengj]

The main point of this, besides a bit of pretty-printing, is to automatically link to github for Julia and Pkg source code (#4952).

Sample output:

[JeffBezanson]

Slick!

[jiahao]

cc: @alanedelman

[ViralBShah]

Is this ready to be merged?

[stevengj]

Yes?

[stevengj]

@StefanKarpinski, it would be good to get your feedback on this regarding url(::Method).

[jiahao]

bump

[malmaud]

This is incredibly useful

[stevengj]

rebased for merge

[StefanKarpinski]

So what feedback did you want from me?

[stevengj]

@StefanKarpinski, basically whether there is something I should be doing differently to get the URL of a method in a package.

[StefanKarpinski]

I man, I dunno. This looks pretty comprehensive. I suspect that we'll have to muddle our way through this one and figure out what's good and what's annoying as people use it. Please don't let me hold this up.

[stevengj]

Okay, merged. We can incorporate other improvements as needed (e.g. @ivarne's suggestion to use Base.BUILD_INFO.fork_master_distance to infer whether one is using a Julia commit that has been pushed to github.)