Refactoring the checking tool
More refactoring to handle text from the web
check.py: fixed PEP8, global errors list

PEP8 changes:
* two lines before function definition
* shortened too long lines (except for one regexp)

Switched to logging:
* now a list of errors for each category is shared between functions

Removed global list of errors:
* instead functions pass a reference to the list that's held in `main()`
* two new formatting helpers: `add_error(msg, errors)` and
  `add_suberror(msg, errors)` -- if you want to know details how they
  work, take a look at their docstrings
Merge branch 'gh-pages' into refactoring-check
Merge branch 'gvwilson-refactoring-check' of github.com:pbanaszkiewicz/workshop-template into refactoring-check
Tidying up @pbanaszkiewicz's improvements to checker