Change the assignment_notebook Bazel rule to accept multiple languages

[salikh]

The data flow of the assignment notebook generation works as follows:

• a single master notebook contains the content in multiple languages (with markers)
  and autograder tests
• multiple student notebooks can be generated from master notebook perl-language
• a single directory with autograder tests is extracted from the master notebook.

Current Bazel rule "assignment_notebook" is capable of extracting autograder tests
and producing a student notebook for one language.

It is cumbersome to add rules for extraction of student notebooks in additional languages,
so the rule should be improved to support multiple languages directly.