Add tests

[maxdeviant]

This PR adds tests to the project.

Resolves #32.

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Checklist:

• Added the change to the changelog's "Unreleased" section with a link to this PR and your username
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation in the README and/or documentation directory
• Added a test for the contribution (if applicable)

[maxdeviant]

I'd also like to add tests for tag, but I couldn't glean how it is intended to be used by looking at the code/docs.

[thomashoneyman]

This is looking good, thank you! I’m on my phone so I will have to come back for a real review tomorrow.

[thomashoneyman]

The tag option lets you take an existing option and fix it to a particular value. For example:

anIntOption :: Option MyOptions Int
anIntOption = opt "anIntOption"

aTaggedInt :: Option MyOptions Unit
aTaggedInt = tag anIntOption 10

Using aTaggedInt will just always set the value of "anIntOption" to 10, and you can only associate with unit (as the argument will be ignored):

aTaggedInt := unit <>
anotherOption ...

I think your existing tests are perfectly adequate, though.