Update 'rake post' to support category on command line

[Miguelos]

Updated comment.
Creating a post with multiple tags require no whitespaces between tags:
rake post title="A Title" [date="2012-02-09"] ... tags=[tag1,tag2]
or quotes:
rake post title="A Title" [date="2012-02-09"] ... tags="[tag1, tag2]"

[marshallshen]

@Miguelos 👍
Can you point me to the Jekyll documentation that allows this option?
Once confirmed that it is an update from Jekyll, I can merge this PR.

Thanks.

[Miguelos]

Here is defined the category variable in the Jekyll documentation http://jekyllrb.com/docs/frontmatter/#predefined_global_variables

In the Jekyllbootstrap documentation: http://jekyllbootstrap.com/api/bootstrap-api.html#list_categories_for_a_specific_post

[marshallshen]

Thanks!