Feat: Add an option tabline_always_visible

[theJian]

Requested feature

Motivation

Related:

#921
#904

For people who rarely use tabline to show anything other than tab pages, keep tabline always on is a complete waste of space. Although there is workaround, it was still quite unexpected that adding this plugin will change user's settings, and having to set options after adding plugins also make it harder to keep configurations clean.

Therefore, I propose adding a new option tabline_always_visible. It's true by default, which behaves as it is now, by setting it to false tabline will be hidden when there are less than two tab pages.

I would like to hear what you think of it. Thanks.

PR: #1137