Add tmux-ram-usage plugin

[aaronkollasch]

This shows the total RAM usage of the tmux server and all child processes. Requires pstree.

Example (in purple):

[ethancedwards8]

We don't generally allow PRs that require the use of an external tool (except in the case where it's immediately obvious that another binary is required like Kubernetes or Git). If you could find a way to do this without the use of pstree I would merge it.

[aaronkollasch]

I added a fallback to get all child pids of the tmux server in case pstree is not installed.

In testing with a fairly small tmux session (2 panes, one neovim process running w/ LSP), the fallback function is slower than pstree on macOS, taking ~2x the time (162 ms vs. 87 ms), but it took about the same time on Ubuntu Linux (38 ms vs. 30 ms). pstree will also scale better than the fallback as it only involves one ps call instead of multiple pgrep calls.

[ethancedwards8]

That's a good compromise, thanks for your contribution!