Update Go 1.19 -> 1.20

With 1.21 RCs coming out, it'd be nice to not be two versions behind. :-)

This also bumps the version to 0.1.7.

Build deb, rpm, and homebrew packages on release

This builds `.deb` and `.rpm` packages automatically when a new tag is pushed.

It also pushes a homebrew formula to the `hrs/homebrew-docsim` repo.

Ensure all Unicode characters are parsed correctly

The previous implementation only handled ASCII characters! There's no reason to
do that---Go supports Unicode just fine, and it's actually *easier* to do the
right thing---so this adds Unicode letter and number support in parsing.

It also extracts token parsing out into a separate function, which makes the
tests more readable.

Since it's now easier to test, this also backfills test coverage for tokens that
include numbers.

Reorganize interface to take default string query

When I started writing `docsim` I'd thought of it as a tool for comparing
documents (hence `doc`ument `sim`ilarity). But I'd been thinking of "documents"
as "files," not as abstract blobs of text including queries, so I added a
`--query` flag that took a file, not a string.

Previously:

- `--query` took a path argument
- passing in a string query only worked through `STDIN`.

With this commit:

- the first positional argument is always a string query, unless either:
  - there's a Boolean `--stdin` flag which reads the query from `STDIN`, or
  - there's a string flag `--file` that takes a file and acts in the current way.

If there's both a `--file` and `--stdin` flag we error and print a usage message.

Usage lines:

    docsim [OPTION...] QUERY [PATH...]

    command | docsim [OPTION...] --stdin [PATH...]

    docsim [OPTION...] --file PATH [PATH...]

This commit also:

- Updates the README and `man` page accordingly
- Bumps the version to `0.1.4`
- Improves usage error message

Extract `hasPermittedExtension` function

Move most logic into `corpus` package

Document the --stoplist flag

Add a build release task