The relative dates are parsed by `numeric_date` which uses datetime.date.today() to translate the relative date to an absolute date.

Relative dates are positive duration values following the ISO 8601 duration syntax
e.g. `--min-date 1Y2W5D` for 1 year, 2 weeks and 5 days ago or `--max-date 1D` for yesterday

This also adds a package dependency `isodate` to parse the duration string.

The previous change to support relative dates also refactored so that an invalid date does not raise any error, which is unwanted.
Prior to that change, there was one try/catch so treetime.utils.numeric_date would handle all these invalid dates.

Explicitly raising from augur.filter.numeric_date is one step in a better direction, though still not a good solution since argparse does not expose these errors.
More: https://stackoverflow.com/q/38340252

- Expose the invalid date error message using argparse.ArgumentTypeError
- Split out the text describing supported dates into a constant variable, since it is used in 3 places now

Previously, some tests for --max-date were not done for --min-date and vice-versa.
Added tests and re-ordered so that similar tests are adjacent.