Fixes #74: Create a shared base for addons.

[miketaylr]

WIP.

[miketaylr]

Mostly happy with this now.

TODO:

• [x] write chrome webpack config, move all those bits.
• [x] write chrome webpack config, move all those bits.
• [x] write commands to build them
• [x] write docs on how commands work

Bonus points to write a command to package up crx, but that seems less important.

[miketaylr]

r? @magsout

[magsout]

@miketaylr it's on my todolist ;)

[magsout]

@miketaylr r+

[miketaylr]

thanks!