Add ability to change exercise-title and solution-title from conf.py …

[yguel]

…and open i18n for those titles.

I made really small changes and it works very well on my tests.
Could you consider the changes for merge ?

• all the best,

[mmcky]

@yguel sorry for the massive delay in reviewing this PR.

We have just done a large scale overhaul of internationalisation in #77 and #75. Does this meet your needs here?

[yguel]

@mmcky no problem for the delay, I know what it is to maintain a tool ... your changes work nicely for me, and that PR can be closed.

For the record, I have archived my PR in the branch i18n_attempt00 in my fork.

However there is still a big problem to fix for translations: the subtitle is not available in the po files as a string that can be translated. I will work on that to propose a new PR based on your work to fix it.