Support `-min-length` flag

[alecthomas]

I feel like it would be useful to only report constants longer than some minimum length. For example, I don't really care if the constant "a" is repeated, but I do care if the constant "Some Long Text" is repeated, as it becomes more prone to error.

[jgautheron]

Good idea, implemented!

[mwat56]

see #7 (Don't count backticks as string members)