v0.1.9: Write bad row info to debug log

We probably want to handle this in a better way, but this will at least
help with debugging.

v0.1.8: Whitespace stripping and column normalization

- `--clean-column-names` will make column names unique and
  database-safe.
- `--trim-whitespace` removes leading and trailing whitespace from
  cells.

v0.1.7: Speed up 2–5%, improve `--help` & modernize

This is a collection of minor help fixes and modernizations, reducing
the total amount of code and speeding things up slightly.

v0.1.6: Add `--drop-row-if-null=COL` flag (fixes #9)

We now support a `--drop-row-if-null=COL` flag which can be used to
automatically discard certain lines when particular columns are NULL.
This argument can be passed more than once to check more than one
column, and it is applied _after_ will normalize NULLs as specified by
`--null`.

In order to implement this, we had to switch from `docopt` (a deprecated
argument parser) to `structopt` (the recommended replacement).

v0.1.5

v0.1.5: Build static binaries