[Feature] Secondary key-binding U removing newlines

[tkapias]

Intro

I'm using Neomutt TUI email client.
The main issue was, for me, that the internal pager does not rely on the terminal to wrap the long Urls.
As explained in this issue, it does insert newlines in lines to wrap them, which break most url extraction tools like the current tmux-fzf-url or the perl script in the URxvt terminal.

Feature

This PR introduce a new feature to work around this issue by removing every newlines in the captured content from Tmux, before the regex parsing.

• This new feature is not an option but a secondary key-binding, by default 'U'.

I separated it in another key-binding because the captured urls might wrongly expand in some cases, e.g, if an Url matches perfectly the screen width and is followed by another string in the beginning of the next line.

But in cases like Neomutt it's a perfect solution.