Shorten mixed indenting warning

[0]

The other whitespace warnings are nice and short, but [mixed-indenting] takes up a bit of room. Could it be shortened to, e.g. [mix]?

[millermedeiros]

thx for the feedback, since my files usually don't have mixed indenting for too long (I do :retab) I end up not worrying about it.

I added an option to configure instead of changing the default value, but now I'm thinking I should also change the default value.

[0]

This issue is really only a problem when working with someone else's code, where changing all the whitespace in a file isn't an option and the whitespace warnings are constantly present.