Show a warning if an image has no alternative text

[brichet]

This PR changes the default alternative text for images, when exporting to HTML, and it also adds a warning in logs.

It also adds test on the warning, and some tests for the language code (which were missing).

EDIT: follows #2021

[blink1073]

Thank you!

[0x26res]

@brichet I've recently updated nbconvert and I started getting warnings.

[NbConvertApp] WARNING | Alternative text is missing on 2 image(s).

So, it makes sense to have these warnings, but I'm not entirely sure how to fix them.

For context I use simple plots from pandas, using `df.plot(title='foo').

From what I can see in pandas's doc and matplotlib's I don't think there's a way to provide an alternative text.

I found an example that overwrite the alternative text, but it's a lot of work: https://gist.github.com/tonyfast/d01a0248979b21c9484d0827e04c0e68

I'm wondering if nbconvert could provide an option to disable that warning?

[brichet]

@0x26res would you mind opening a separated issue for it.
It should probably be discussed as the issue mainly come from matplotlib and pandas.