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?