Disable the progress bar without disabling other pretty things

[eldipa]

Describe the feature you'd like
When running byexample as a pre-commit hook as proposed in #202 , the output looks almost perfect:

**********************************************************************
File "Foo.md", line 4
Failed example:
    1+2
Expected:
3a
Got:
3

File Foo.md, 3/3 test ran in 2.02 seconds
[FAIL] Pass: 2 Fail: 1 Skip: 0

File README.md, 3/3 test ran in 2.01 seconds
[PASS] Pass: 3 Fail: 0 Skip: 0
README.md: 100% 3/3 [00:02<00:00,  1.49it/s, line 5]

The last line however looks weird. The README.md: 100% 3/3 [00:02<00:00, 1.49it/s, line 5] is just an artifact of the progress bar of byexample.

This progress bar is not shown by pre-commit because it executes byexample in background and only prints its output at the end.

For this reason byexample should have a way to disable the progress bar. In fact it has one with --pretty none but that also disables the colors.