add wp-cli command

Conflicts:
	jekyll-export-cli.php
	readme.md
better WP-CLI, non-WP-CLI-compatability
check if plugin is activated via CLI
Merge pull request #19 from benbalter/wp-cli

Add wp-cli command, props @scribu
Merge branch 'master' of https://github.com/benbalter/wordpress-to-jekyll-exporter into the-return-of-markdownify

Conflicts:
	jekyll-export-cli.php
	jekyll-export.php
	readme.md