Add calendar command to see in which days activities were logged

[JacobEvelyn]

I'm not yet sure what this should look like exactly, but I think it would be useful to see something like:

$ friends calendar
     June 2017
Su Mo Tu We Th Fr Sa
             X     X
 X  X  X     X     X
 X     X            
    X           X  X
    X  X     X

And, similar to friends graph, you could add flags to filter what you see, e.g. friends calendar --in Boston --with Prudence

Some questions:

1. How should "active" days be displayed?

• With a symbol, like an "X"?
• With a number indicating the number of activities?

2. How should "inactive" days be displayed?

• With nothing (as above)?
• With the day of the month?
• With a symbol, like an "x"?

3. How should days that have not yet occurred be displayed?

• The same as other days?
• The same as other days but a different color?
• Not displayed at all?

4. Should multiple months be displayed at a time? If so, how? If not, how do we specify what to show?
5. Should this be the way that a user can see their "streak" of consecutive days, or would that better fit as a separate feature?

I'm interested in hearing opinions from anyone!