Add channel property to browsers

[chicoxyzzy]

The problem

Firefox SIMD support [4] note

Firefox 52 Nightly should have yes results in 56 of 57 tests for SIMD.

Maybe we should add a property nightly to Firefox 52 info. Something like

  firefox52: {
    full: 'Firefox',
    short: 'FF 52 Nightly',
    unstable: true,
    channel: 'nightly',
  },

...so we'll be able to set test results to yes in the table for tests with res having firefox48: firefox.nightly

[afmenez]

That would be nice. I am having to add a new "firefox.developer" note for a feature that was just "promoted" (atomics). It would be also nice to create a "beta" channel for Firefox, and a "preview" channel for Safari and Edge.

[afmenez]

I am not sure these results should be shown as a plain "yes", which could be misleading. I believe something more like a flagged issue would be more appropriate.

[chicoxyzzy]

@afmenez I think all features that available without any flag should be marked as "yes". We can still add footnote to make it clear that these features won't land in other channel in next release.