Adds spell checking

[codingjoe]

What this PR adds:

• without installed aspell / enchant the tests go as usual, all the translation spellchecking tests are just skipped. (you see the skipping then)
• when you install the corresponding aspell dictionaries, and enchant (see the new readme), the items are not skipped any more, and you should see the spelling errors
• you can set an environment variable to a directory, to provide private wordlists for each language (also see the readme)

[syphar]

• tests fail
• IMHO the spellchecking should either be optional, or skip (with a warning) checking a language if the dictionary is not found.

[syphar]

• tests fail
• couldn't test further, since I couldn't install the required packages.

[codingjoe]

Ok, I should have caught all your comments.

[codingjoe]

The dicts for multiple languages are still missing.

[codingjoe]

@syphar I added multiple language support for private word lists and python format string chunkers. Ready when you are :)

[syphar]

I also see

SKIP [5] ~/src/pytest-translations/pytest_translations.py:322: Dictionary "" not found.

Reason was that the Language attribute in some of the .po files was missing.

I think we should fail in that case. Or assume english. But I tend to fail.

[syphar]

@codingjoe you can have a first look

[normansander]

A description would be helpful

[syphar]

@headione you mean about what the PR should change / do? or general documentation?

[normansander]

yes @syphar, i was missing a little description in this pull request and its impacts. for me it was a bit hard to understand the feature without it, although it has a precise title. but other than that LGTM 👍🏼

[syphar]

@headione added a description

[codingjoe]

@syphar I'm happy except the missing new line? Is there anything you'd like to add before I merge?

[syphar]

@codingjoe newline fixed.

If you tested and everything is fine, feel free to merge