Add docs for i18n and unify debug-locales to run per app

[muffinresearch]

Fixes mozilla/addons#9619

• Unifies all the locale commands to require NODE_APP_INSTANCE
• Adds docs for the commands

[coveralls]

Coverage remained the same at 100.0% when pulling 8ddc896 on muffinresearch:add-docs-for-i18n into 0966fc7 on mozilla:master.

[coveralls]

Coverage remained the same at 100.0% when pulling c6fb852 on muffinresearch:add-docs-for-i18n into 0966fc7 on mozilla:master.

[mstriemer]

I know this is old but it's always bugged me. I'd rather see something like:

const debugLocales = [
  ['dbl', 'unicode'],
  ['dbr', 'flipped'],
];
for ([locale, rewrite] of debugLocales) {
  // ...
}

[muffinresearch]

Looks like there's not enough es6 capability to do this since these are just node scripts.

[mstriemer]

Thanks for documenting it! r+wc

[coveralls]

Coverage remained the same at 100.0% when pulling 66588ad on muffinresearch:add-docs-for-i18n into c254967 on mozilla:master.