Search using regular expression string

[nakagami]

• https://github.com/prompt-toolkit/python-prompt-toolkit/blob/master/src/prompt_toolkit/key_binding/bindings/search.py#L37-L44
• https://github.com/prompt-toolkit/python-prompt-toolkit/blob/master/src/prompt_toolkit/search.py#L183-L214
• https://github.com/prompt-toolkit/python-prompt-toolkit/blob/master/src/prompt_toolkit/buffer.py#L1501-L1518

and related to :set ignorecase and :set ic ignored prompt-toolkit#147

[nakagami]

find()
https://github.com/prompt-toolkit/python-prompt-toolkit/blob/master/src/prompt_toolkit/document.py#L360-L399

find_backwards()
https://github.com/prompt-toolkit/python-prompt-toolkit/blob/master/src/prompt_toolkit/document.py#L409-L436

[nakagami]

Search formard using regular expressions is working, now.
The regular expression \< \>, which is specific to vi, cannot be used because the python re module is used, but this is a specification.

Search barckwards using regular expressions still not working.