This adds an `excludeExamples` option, defaulting to `true`.
When enabled (or not configured) it will mask all `@example` tags
and their content before testing indentation.
That prevents errors about indentation inside example code
blocks and keeps correct line numbers when reporting indentation error
found elsewhere.

It should fix gajus#334.

Ideally this would default to `true`, but that could break existing setups.

Instead of rather specific `excludeExamples` option, we now have more
generic `excludeTags` option. It takes an array of tag names to exclude
from `check-indentation`, and defaults to `['example']`.

Added test suggested by @golopot:
gajus#388 (comment)