More performant BasicAuth plugin

[domanchi]

Summary

This should make the BasicAuth plugin much more performant, and fixes #79 and #77.

Testing

$ time python detect_secrets/main.py scan --no-hex-string-scan --no-base64-string-scan --no-keyword-scan --no-private-key-scan <large-file>

It actually finishes this time! Furthermore, I modified the large file by inserting a secret that I'd expect it to find, and it finds it as well.

Rationale

We're already doing BASIC_AUTH_REGEX.findall(line) which will run the regex on all non-overlapping matches of the pattern in the string. This means that we can ignore the prefix catch-all expression (.*?) because we don't care about that, and we can let the python library handle it for us.

[KevinHock]

LGTM!

[joshuarli]

Nice one @domanchi!