Case-insensitive history search

[tailhook]

I'd like to have case-insensitive search of the history. And it looks like history search is hardcoded now. How should I approach the problem:

1. Allow customizing history search?
2. Add case_insensitive (or similar) config parameter?
3. Make case insensitive default (if that makes more sense)?

[gwenn]

If possible we should behave like readline which seems to be case-sensitive.
So (2) seems good to me.
I will try to see how other libraries behave by default.

[gwenn]

It seems that we need the regex crate (rust-lang/rfcs#1988).
And we cannot use std::str::pattern (yet): https://docs.rs/regex/1.1.0/regex/struct.Regex.html#using-the-stdstrpattern-methods-with-regex

[gwenn]

See #546 but you may not like the cargo feature...

[tailhook]

I'm fine with it being cargo feature. I think that case-insensitive here is what users would expect, and it's fine that it's a feature because of having an extra dependency.

[gwenn]

Version 9.0.0 released.