Add tabusevisible switchbuf option

[igorlfs]

Addresses #1470

I've been using this for a couple of weeks without any issues. But I've been holding back onto opening the PR because I don't think that's a long-term solution: adding variations to the switchbuf option is awkward.

You'd think that this would be the last one, but we can go further. For instance, the usetab value does not take into account if there's a window where the line is visible (akin to the usevisible option) within the same tab. Instead, with usetab, the first window that matches the buffer is always used, regardless of the line being visible elsewhere.

Thoughts?

[mfussenegger]

I'm also not sure where to draw the line here - currently I'm tending more towards simply allowing switchbuf to be a function and let users customize it as they see fit.

Also given issues like #1478
Accounting for all the different buftypes/layouts/behaviors that people might want seems to be a never ending story otherwise.