Pypistats

* Add `--monthly` option: https://github.com/hugovk/pypistats/pull/38
* Add local caching of API calls: https://github.com/hugovk/pypistats/pull/39
* Fix ValueError: month must be in 1..12: https://github.com/hugovk/pypistats/pull/41

Instead of returning `None`, raise an exception when a bad request was made (a 4XX client error or 5XX server error response)

Fix a bug:

* Comma-separate the thousands in `recent` tables, like the others

Before
console
pypistats recent pip
| last_day | last_month | last_week |
|---------:|-----------:|----------:|
| 1799469 | 67307635 | 15490986 |


After
console
$ pypistats recent pip
| last_day | last_month | last_week |
|----------:|-----------:|-----------:|
| 1,799,469 | 67,307,635 | 15,490,986 |

Added `--format` which can take `markdown` (the default), `json` and new formats `rst` and `html`.