Add support to passing variables as command-line options

[andrioni]

This implements the option to pass variables to playbooks using command-line arguments.

• -var key1=val1 gets passed as the variable key1 having the value val1.
• Multiple key-value pairs can be passed by just repeating -var: -var key1=val1 -var key2=val2
• Command-line variables override those defined in the playbooks.
• The first equals sign (=) found marks the separation between key and values: this means a value can contain as many equals sign it wants to, but the key can't.

[alexanderdean]

Awesome implementation Alessandro! Thanks so much, scheduling.

@andrioni has signed the CLA.

[andrioni]

Happy to help!

[alexanderdean]

How are you finding Golang?

[andrioni]

This was my first experience writing anything other than a Hello World, and it was pretty nice.
I like the straightforward simplicity, and it feels refreshing to do something so down-to-earth after spending some months in Scala-land.

[neekipatel]

Hi, any idea when you will release the next version. Waiting for this to be merged as writing the db creds for every playbook is a pain.

[alexanderdean]

Not too long now @neekipatel - thanks for your patience...