Command to sign web extensions

[kumar303]

Fixes #145

NOTE: this depends on mozilla/sign-addon#14 which also needs to be released to NPM before the tests in this pull request will pass.

signing works!

$ web-ext sign --api-key ... --api-secret ... --api-url-prefix https://addons-dev.allizom.org/api/v3
Created temporary directory: /var/folders/h7/rttdqkks7fl_txp9_rr4j4840000gn/T/tmp-web-ext-25722PO7exnQqnPrc
Validating manifest at /Users/kumar/dev/bookmarker-ext/moz-webext/manifest.json
Building web extension from /Users/kumar/dev/bookmarker-ext/moz-webext
Using manifest [email protected]
Not including file /Users/kumar/dev/bookmarker-ext/moz-webext/.DS_Store in ZIP archive
Not including file /Users/kumar/dev/bookmarker-ext/moz-webext/web-ext-artifacts/getting_started_example-1.2.19-fx.xpi in ZIP archive
Not including file /Users/kumar/dev/bookmarker-ext/moz-webext/web-ext-artifacts/getting_started_example-1.2.21-fx.xpi in ZIP archive
Your web extension is ready: /var/folders/h7/rttdqkks7fl_txp9_rr4j4840000gn/T/tmp-web-ext-25722PO7exnQqnPrc/getting_started_example-1.2.22.xpi
Validating add-on [..........................................................................................................................................................................................................................................................]Validation results: https://addons-dev.allizom.org/en-US/developers/upload/8865b96e71f942f5940b4cba0ee3825b
Downloading signed files: 100%
Downloaded:
    ./web-ext-artifacts/getting_started_example-1.2.22-fx.xpi
SUCCESS
Removing temporary directory: /var/folders/h7/rttdqkks7fl_txp9_rr4j4840000gn/T/tmp-web-ext-25722PO7exnQqnPrc

[rpl]

Nit, how about assigning stubs.signAddon.firstCall.args[0] to a shorter variable name (e.g. let signAddonArg = ...) to make this sequence of assertions a bit more readable?

[rpl]

looks good to me

NOTE: tests are currently failing on travis because the sign-addon package is not yet on the npm registry, but they successfully complete locally, by linking the sign-addon into the local node_modules dir.

[coveralls]

Coverage increased (+0.08%) to 99.029% when pulling 95c2d50 on kumar303:sign into a13ac13 on mozilla:master.

[kumar303]

The tests are passing after I published sign-addon. Thanks for the review!