Close #1197 ember build --watch should run until SIGTERM
Update --dry-run, add --skip-npm and --skip-bower

Previously --dry-run meant a couple different things.

In the `init` command it meant “don’t run npm install or bower install”.
In the `generate` command it meant “don’t overwrite any existing files”.

This patch changes the behaviour of the flag in both places.

--dry-run now only ever means “don’t write *any* files”

Two new flags are introduced for `new` and `init:

--skip-npm
--skip-bower

Their behaviour is hopefully self-explanatory.

Addresses #1199
Merge pull request #1205 from jgwhite/blueprint-dry-run-patch

Update --dry-run, add --skip-npm and --skip-bower
Merge pull request #1198 from tabolario/bugfix-1197

Close #1197 ember build --watch should run until SIGTERM
remove .gitkeep from ember init updates
Merge pull request #1209 from jakecraige/feature/no_gitkeep_on_update

remove .gitkeep from init updates
lookupCommand now checks in addons for a command if it's not in core
extract logic to get a projects addonCommands to a method on the project
move addonCommand to a fixture to reuse it
add addon commands to help and jshint fix
pass project into lookupCommand when running cli command
update changelog
fix tests, style update
log warning when addon tries to override a core command
verify dependency package.json path exists before requiring it
Fix new command not running from cli since pkg was undefined
Group help commands output by addon
add eachAddonCommand method to Project to simplify looping over addonCommands when it's done
Merge pull request #1196 from jakecraige/feature/addon_commands

Feature: Addons add commands to the CLI
Do not watch the same directory more than once.