Ncflag

- use long_description_content_type="text/markdown" in setup.py
- unpin click dep, tested with latest 8.1.3

- `ignore_missing` option on `get_flag` to bitwise and flag meanings, but not raise exception if a
meaning is missing from the flag.

- New method: `FlagWrap.is_valid_meaning` for convenience.
- Change reduce method to exclude _entire_ flag vectors (all bits) from reduction if any bits in
the exclude_mask are set.
- Make FlagWrap.flag_meanings "public" (prev. FlagWrap._flag_meanings)
- Adds name attribute, available from init as kwarg, for misc use.
- Drops support for Python 2.7

- Bug fix: cli: fix `--show_flags` option not working for Python 3

- cli: Support Python 3 by removing relative import.
- setup.py move to `convert_file` from deprecated pypandoc `convert`
- Adds shape, fill shortcuts for `init_from_netcdf`
- Persist internal reference to netcdf object for convenience in `write_to_netcdf`.

- netcdf agnostic, just need flag_meanings, flag_values, and optionally
flag_masks.
- Previous FlagWrap(netcdf.Variable) construction should be replaced with
FlagWrap.init_from_netcdf(netcdf.Variable) class method to conveniently
construct from FlagWrap.
- Confusing sync method removed. Replaced with interface:
FlagWrap.write_to_netcdf(netcdf.Variable).
- Fixed bug in setting mutually exclusive flag_meanings.
- Added comprehensive testing.