Add hashtag capabilities to activities

[JacobEvelyn]

It would be nice to be able to do something like this:

friends add activity "Went for a run. #exercise"

and then later filter activities like:

friends list activities --tagged exercise

or

friends list activities --with James --tagged restaurants

For now this would only support one tag to be passed to the --tagged argument (see #88 for more details on lifting that limitation).

[andypearson]

I'd like to be able to tag friends as well, so I can search for people that I worked with, or people I know from school.

[JacobEvelyn]

Ah interesting, so you're saying there might be friends under the #work tag as well as activities under the #work tag. That makes sense to me. I just created #90 for this; let me know if that looks like what you were thinking.