Make Ahocorasick matching case insensitive

[zricethezav]

This PR fixes a bug that prevents secrets from being detected if the keyword or chunk data case varies.

strings.ToLower doesn't seem to have a huge effect on performance either.

Benchmark git scan on the rails/rails repo

### NEW
real 27.18s
user 58.14s 
sys  5.47s 
memory:753328KB 
cpu 234%
      62

### OLD
real 27.45s
user 56.43s 
sys  5.50s 
memory:750160KB 
cpu 225%
      62

To demonstrate this bug fix, try finding a private key

[mcastorina]

I'm not sure we have any non-lowercase keywords, but good to have this in place.

[zricethezav]

@mcastorina I didn't see any, but I agree. Currently, some secrets like private keys are being detected because https://github.com/BobuSumisu/aho-corasick does not have a case insensitive option like https://github.com/petar-dambovaliev/aho-corasick and therefore when BEING PRIVATE KEY is scanned it fails to match on private key (pkey detector keyword).

Note that the performance hit in the test is a tradeoff for correctness.

Perhaps we can follow up with an option in the library to support case insensitive scanning