Fix nested markup and add tests

[kovariance]

The main purpose of this PR is to add some unit tests. Not a lot (~8% coverage). Along the way, I discovered a couple of bugs.

Firstly, nested inline markup did not parse as expected. Evaluating parsetree(org"*/text/*") returned the following:

Org Parse Tree
    Section
        Paragraph
            TextBold
                TextPlain "/text/"

It now returns the following:

Org Parse Tree
    Section
        Paragraph
            TextBold
                TextItalic
                    TextPlain "text"

Secondly, the TextMarkup(::Symbol,::String) constructor was functionally broken. Granted, this constructor isn't documented.

[tecosaur]

Thanks for this work! I've recently started the long-promised rewrite though, so I'm afraid the code you've made this PR against will be gone in the next version of the package 😅

[kovariance]

Thanks. I am going to close this then. Did not know about the rewrite. If you'd like, I'd love to contribute to the test suite. Writing unit tests is how I achieve zen.