Add YAML support with read_yaml

[nejch]

I just found this neat little project looking for some YAML -> md conversions for mkdocs and this PR would make life easier for me :) Closes #17.

A few things I wasn't sure of:

• I added pyyaml to install_requires, not sure if this should be an optional extra_requires and installed as mkdocs-table-reader-plugin[yaml]?
• I only pass the filename to open(), I couldn't imagine people doing anything fancy here. I do pass **kwargs to pandas though.
• Maybe for a future PR, but pandas defaults to NaN (numpy.nan) for missing values in the table, which is kind of ugly. It might be nice to have a default_cell_value="" arg and replace it. People could then use N/A, 0, etc). Also since YAML is usually more of a machine format it might be nice to have something like capitalize_headers=False, so people could get prettier header titles (e.g. with string.capwords()). I tried both of these and it should work, but not sure if it's in the scope of this PR.

[timvink]

Hi @nejch,

Thanks for the contribution, looks good!

• Thought about making yaml optional, but this plugin already relies on pandas which is pretty heavy, and yaml is quite common. Let's keep it.
• No need to support fancy use cases (liked more nested structures) in this PR, open to it in the future if someone needs it.
• I'm open to a PR that allows for some more options. You could set them in the command ({{ read_yaml('file_path', missing='' }}) or as a plugin parameter (read_yaml__missing: "")

I will add some extra references to other mkdocs packages that work with yaml, and when to use them, and release right after.