Basic bindkey functionality

[llloret]

Inspired in readline's rl_bind_key functionality.

So that in the application we can do something like:

    linenoiseBindkeyAdd(ctrlChar('T'), &f1);
    linenoiseBindkeyAdd(ctrlChar('X'), &f2);    

And call those functions when the keys are detected while in linenoise()

[llloret]

Hi, is there any interest on this PR?

[jsteemann]

Yes, and thanks!
We would be happy to accept the patch, but for legal reasons we need you to fill out and sign our Contributor Agreement. You can scan and email the pdf file to cla@arangodb.com or send it via fax to +49-221-2722999-88. Sorry for the inconvenience, but once the agreement is there the PR can be merged.

[llloret]

Where can I find the Contributor Agreement?

[jsteemann]

Sorry, I didn't send the link. The CLA PDF is here: https://www.arangodb.com/documents/cla.pdf

[fceller]

@llloret thanks a lot for your fix. Can you sign a contributor licence agreement available at https://www.arangodb.com/documents/cla.pdf?