Specify alias file as a command line argument

* Add a dependency on 'getopts'
* Remove the hard-coded "testaliases" file used previously.
* write_alias: Update to include a `file` attribute that can reference
  an arbitrary file
* find_alias_in_file: Change the `file` parameter to be a Path reference
  instead of a string so that it can be called correctly from
  `write_alias`. Also because it matches the File module's signature.

Originally I planned to make the file argument available under a `-f`
command-line option. Later I decided instead to make it a required
argument, so it made more sense not to prefix it with an option flag.
Since I no longer need command line options—just the first argument—I
realised that I could get rid of the `getopts` dependency and use
`std::env::args`. Will do this in a later commit.

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