Add support for attaching tags at runtime over the CLI

[alexanderdean]

No description provided.

[ninjabear]

Advanced to 0.3.0, this feature is essential as part of #59

[ninjabear]

I've added a command line option --tag=key,value that has the following effects:

1. it's used to calculate the "job reference" (SHA256) hash (including the Factfile as normal)
2. all tags are passed through in the webhook as a list of key -> value pairs under a "tags" object, e.g.

$ factotum run samples/echo.factfile --webhook http://*** --tag="foo,bar" --tag="foo2,bar2"

results in the following structure being inserted at the root level of the webhook update json (job and task level)

...
"tags":{"foo":"bar","foo2":"bar2"}
...

[alexanderdean]

Perfect!