Add _linting-in-progress_ (subtle) visual indicator in the statusline

[bew]

Since mypy (or other linters) can take some time (for mypy: until #58 is fixed though), it's not always clear if the linters runs are still in progress or not.

The indicator could be placed here, to avoid too much visual left/right flickers:

[bew]

See :h neomake-statusline and related functions

[bew]

Symbol ideas:

•  (a checkmark) for saying "everything is ok"
• Either .. or a braille spinner? Example braille spinners:
  • ⣾ ⣽ ⣻ ⢿ ⡿ ⣟ ⣯ ⣷
  • ⠇ ⡆ ⣄ ⣠ ⢰ ⠸ ⠙ ⠋
  • ⡀ ⡄ ⡆⠇⠋ ⠙ ⠸ ⢰ ⢠ ⢀ ⢀ ⢠ ⢰ ⠸ ⠙ ⠋ ⠇ ⡆ ⡄ ⡀ (spinning slowly, to avoid stealing my attention too much?)
• Basic format E2 W4 to show error / warnings (after the run/spinner symbol if a job is still in progress)

Question: to make a spinner, how to trigger the statusline to re-calculate itself? can each segment have an independent update cycle? or do we need to calculate the the current time / do something 'complex' to know which spinner cycle to use?