When specifying a `--version` for a plugin, always install that version.

Previous behavior was to quit if the plugin was already installed.
Merge branch 'master' into force-install-476
Improve `theme install` logic to support force-installing a specific version.

Previous `theme install` logic:

* Updates the theme if there's an update available.
* Installs if the theme isn't installed.
* Otherwise, assume the theme is installed.

New `theme install` logic:

* Don't do anything if theme is already installed and no `--version` specified.
* If `--version` is set and is different than what's installed, force install.
* Install if the theme isn't installed.
A Behat test for force-installing a plugin
Behat test for force-installing a specific version of a theme.
Mention why there's no break

See https://github.com/wp-cli/wp-cli/pull/476/files#r4981709

[ci-skip]
If a plugin or theme is already installed and the `--version` parameter is used, prompt the user to confirm they'd like it installed.

See https://github.com/wp-cli/wp-cli/pull/476#issuecomment-20354895
Update stories to support b07154e. Behat isn't working properly on my local machine bcause of my shoddy connection, so let's see if this passes in Travis
Force install Akismet, as it comes with a default WordPress install.
Merge branch 'master' into force-install-476

Conflicts:
	features/theme.feature
	php/commands/plugin.php
	php/commands/theme.php
replace --yes with --force
document --force flag
Merge branch 'master' into force-install
leverage 'clear_destination' parameter from WP_Upgrader
simplify logic for theme and plugin installs
Merge pull request #476 from wp-cli/force-install-476

Support for `--force` flag when installing plugins or themes