[liquid_tags:notebook] Add language highlighting option

[isms]

Hey @jakevdp -- here's a random pull request for liquid tags! ✨

This adds an option to specify the language which Pygments should use for highlighting code cells.

{% notebook filename.ipynb language[julia] %}

This may be helpful for those using IJulia or notebooks in any other language, especially as the IPython project broadens its scope of language compatibility. By default, the language for highlighting will be ipython.

This came up for me last night, hence the pull request. It's working:

Also:

• Updated Readme.md for this and the cells option
• Fixed a couple other small typos elsewhere in Readme.md
• Add tests for notebook tag regex

$ python -m liquid_tags.test_notebook
.......
----------------------------------------------------------------------
Ran 7 tests in 0.000s

OK

[jakevdp]

Looking at the regexp, it will only work together if cells is specified before language. Making it agnostic to order is hard with regexps; the easiest fix would be to just update the docs here.

[jakevdp]

I've not tried this, but it looks pretty straightforward.

[isms]

@jakevdp - Great point in #283 (comment), fixed by 2fa1551

[justinmayer]

@jakevdp: Thanks for chiming in so quickly. (^_^)

@isms: Thanks for the contribution. Would you please squash the latter two commits into the first? (i.e., resulting in a single commit)

[isms]

@justinmayer: All set, I think. Let me know if I missed anything.

[justinmayer]

Many thanks, Isaac.