Allow yyyy-mm for -sd & -ed, add --month shortcut

[hugovk]

Fixes #61.

• Allow yyyy-mm for --start-date and --end-date, which map on to the first day of the month for --start-date and the last day for --end-date.

• Add --month as shortcut to --start-date and --end-date for a single month

These three are equivalent:

$ pypinfo --test --start-date 2018-04-01 --end-date 2018-04-30 setuptools
$ pypinfo --test --start-date 2018-04 --end-date 2018-04 setuptools
$ pypinfo --test --month 2018-04 setuptools

Each outputs:

SELECT
  COUNT(*) as download_count,
FROM
  TABLE_DATE_RANGE(
    [the-psf:pypi.downloads],
    TIMESTAMP("2018-04-01 00:00:00"),
    TIMESTAMP("2018-04-30 23:59:59")
  )
WHERE
  file.project = "setuptools"
  AND details.installer.name = "pip"
ORDER BY
  download_count DESC
LIMIT 10

[codecov]

Codecov Report

Merging #71 into master will increase coverage by 4.22%.
The diff coverage is 97.61%.

@@            Coverage Diff             @@
##           master      #71      +/-   ##
==========================================
+ Coverage   66.14%   70.37%   +4.22%     
==========================================
  Files           5        5              
  Lines         257      297      +40     
  Branches       35       35              
==========================================
+ Hits          170      209      +39     
- Misses         87       88       +1

Impacted Files	Coverage Δ
tests/test_core.py	100% <100%> (ø)	⬆️
pypinfo/core.py	61.29% <95%> (+3.55%)	⬆️

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[hugovk]

Will merge this next month if no objections. Thanks!