Add repository activity as time series

Create report with repository activity for each month from date range:

``` YAML
command: repo_time_series
date:
from:2012-03
to:3012-05
```

**Output:**
repository, month1, month2, month3
user/repo, 1,13, 3

Start by locating the command or report entry point for `repo_time_series` and reviewing how existing reports handle date ranges. Done means the command accepts the shown from/to months and produces one repository row with activity values for each month in the requested range.

Written by the indexing model from the issue text.