make default plugins configurable

[ExpandingMan]

The default provided bars are useful but extremely verbose. Currently the only option is to remove them entirely (frankly this almost works well for me but the lack of a mode indicator of any kind is very confusing).

This PR seems to address this but I'm not sure what the status is.

[imsnif]

I think @a-kenji made a pretty cool plugin that displays the mode along with the tab bar. Maybe if he has the time he can put it somewhere?

[ExpandingMan]

A link to that would be much appreciated, thanks.

[a-kenji]

I will try to make it easily accessible. It is basically the tab-bar slightly modified.

[a-kenji]

@ExpandingMan
I commited it to a branch of my repo.
You can directly download it here:
https://github.com/a-kenji/zellij/blob/tab-bar-modes/assets/plugins/tab-bar.wasm

Or build it from the branch.

• The tab-bar shows the current mode after the session-name
  This is a very slight change, intended to bridge the gap until
  plugin configuration lands.

  How to use:

  Copy tab-bar to the platform specific plugin location,
  if in doubt run: zellij setup --check

  cp assets/plugins/tab-tab.wasm  ~/.local/share/zellij/plugins/tab-bar.wasm
  

  Warning! This is based on the current master, if zellij-tile-utils changes
  this could break.