fix --rules-dir in mathml2text CLI

[moritz-gross]

While testing out the mathml2text CLI, I noticed that --rules-dir seems to not actually be used.
For example, the following shouldn't work right? cargo run --bin mathml2text -- --rules-dir /some/other/rules

Regarding the integration tests, I don't really know what I'm doing there.

[masonium]

@NSoiffer asked me to take a look. The actual rules_dir fix looks good. I'll leave a couple of other comments inline.

[NSoiffer]

@masonium: Should I close this without merging?

[moritz-gross]

I think the comments by @masonium make sense, I just forgot that this PR existed. I'll do it in the coming days.

side note: I'm unsure how I just update this branch. 99% sure I clicked "Sync fork" in my repo, so https://github.com/moritz-gross/MathCAT should've been affected. or what am I missing?

[moritz-gross]

@masonium is this what you had in mind? so far I've not written any fixtures for CLI or file system interaction, particularly in Rust.

Is the Drop trait needed when using temp_dir? From how I unterstood the docs, it is, but I'm usure.

failure of linting in main.rs seems unrelated to my changes.