[REVIEW ONLY] Prototype launching browser via node domain

This is a _REALLY_ early prototype for allowing users to specify their own path to Chrome/Chromium and maybe even Firefox, for use with live preview.

TODO clean up Firefox instructions

To get FirefoxAurora working:

1. Follow instructions from https://github.com/auchenberg/remotedebug-firefox-bridge#getting-started and https://github.com/auchenberg/remotedebug-firefox-bridge/issues/3
2. Once remote debugging is on, open this URL manually `file:///Applications/Brackets.app/Contents/dev/src/LiveDevelopment/launch.html`
3. In Brackets, open live preview

I could only get live HTML to work, but not live CSS.

To use Chrome Canary:

1. Toggle `isDefault` flag https://github.com/adobe/brackets/pull/6622/files#diff-c9d3abef7253cef9bb29540cd3bd87faR78
2. Clear current `isDefault` settings

Launching 
- [x] Replace our `brackets.app.openLiveBrowser` and `brackets.app.closeLiveBrowser` with a node domain that uses `child_process` to execute programs from node

Preferences
- [x] Remove hardcoded Mac paths
- [x] Setup basic prefs for Chrome/Canary/FirefoxAurora
- [ ] Save prefs to `user` scope for easier access @dangoor 
- [ ] Do not clobber `user` scope prefs if they already exist

UI
- [ ] Display browsers in a menu?
- [ ] Set default browser, see `isDefault` flag

Populate default browsers at first launch
- [x] Windows - use `REG` to find path to `chrome.exe`
- [x] Mac - use `mdfind "kMDItemCFBundleIdentifier == 'com.google.Chrome'"` to find Chrome
- [x] Mac - use `codesign --display /Applications/Google\ Chrome.app` to get to binary in app bundle
- [ ] Linux - Find path to exe or just rely on `$PATH`?

Code cleanup
- [ ] Make `ChildProcess` API match node `child_process`
- [ ] Error handling