Add ability to specify HMR hostname

[suevalov]

I've got a problem after #426 being merged. I'm using parcel for Chrome Plugins development and cannot rely on window.location.hostname value.

Url of the page i'm working on is chrome-extension://<extension-id> and HMR doesn't work, because window.location.hostname is equal to <extension-id>.

This PR adds new hmrHostname option. If it's not set, window.location.hostname is being used.

[suevalov]

Any update or feedback?

[devongovett]

You probably need to add this to the CLI as an option too, right? https://github.com/parcel-bundler/parcel/blob/master/src/cli.js#L16-L20

[codecov-io]

Codecov Report

Merging #492 into master will decrease coverage by 0.31%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #492      +/-   ##
==========================================
- Coverage   88.41%   88.09%   -0.32%     
==========================================
  Files          56       56              
  Lines        1640     1638       -2     
==========================================
- Hits         1450     1443       -7     
- Misses        190      195       +5

Impacted Files	Coverage Δ
src/Bundler.js	88.88% <ø> (ø)	⬆️
src/packagers/JSPackager.js	100% <ø> (ø)	⬆️
src/utils/config.js	92.85% <0%> (-3.58%)	⬇️
src/transforms/postcss.js	93.33% <0%> (-0.22%)	⬇️
src/transforms/posthtml.js	100% <0%> (ø)	⬆️

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[suevalov]

@devongovett Good catch, thanks. Added.