.tmcignore specify course

[josalmi]

It would be nice to be able to specify which course .tmcignore file concerns.

This currently hidden course: https://tmc.mooc.fi/hy/courses/42 should be a good example. Now we have exercises from week 1 to week7 listed as hidden when we only want to show weeks 8 to 14 to users.

Possible fixes:

• If .tmcignore contains text lines contain course names that the rule applies to. One course name per line.
• .coursename.tmcignore, probably bad idea because course names might contains some special characters? But would be fast to check. Possibly lot's of ignore files.
• Get rid of .tmcignore and use metadata.yml ignore: true

[jamo]

This should be done with course templates, not anymore in the config files.