Support default Tmux configurations

After writing this, I have a sneaking suspiscion that I'm reinventing
something like Tmuxinator. I guess this is just a poor man's Tmuxinator.
For now, however, haven't had enough time to look into Tmuxinator as a
viable alternative, so this will have to do for now.

Furthermore, moved root-level tmux files to a subdir.

tmux/default.tmux

@@ -0,0 +1,14 @@
+# Simple script designed to be `source-file`'d from within Tmux. It will setup
+# panes with common directories for work.
+
+# Dotfiles
+new -s dotfiles -n main
+send-keys 'cd ~/dotfiles' C-m
+
+# Meta
+new -s meta -n main
+send-keys 'cd ~/urbint/meta' C-m
+
+# Grid
+new -s grid -n main
+send-keys 'cd ~/urbint/grid' C-m