Add 'bind' keyword argument

[Gregory-K]

The key/event bindings to pass to 'fzf'.
Dictionary of the form {KEY: ACTION} or {EVENT: ACTION}.
Corresponds to --bind=KEYBINDS option.

example:

keybinds = {
    "ctrl-j": "accept",
    "ctrl-k": "kill-line",
}

iterfzf(iterable, bind=keybinds)

Related Issue #21

Notes:

• Pull-Request includes a minor fix: 'Annotate 'case_sensitive' as Optional explicitly (according to the general code style/format)'. Please provide guidance on including minor fixes or not.

• The keyword argument 'bind' was placed according to fzf's manual 'categories' under 'Interface'.

• A 'bind' kwarg text snippet has been inserted in the README file. Uncertainty existed regarding the preferred writing style and whether it was appropriate to update the 'Changelog' section.

Sub-Notes:

It is inferred by code comments that the fzf manual 'categories' are the desired grouping/sorting of the kwargs. If the hypothesis is true, iterfzf() keyword arguments and their listing in README should be rearranged and match.

[dahlia]

Could you add an entry for this change to the changelog? The changelog is at the bottom of the README.rst.

[dahlia]

Thank you for your contribution!