Add tmux fzf session chooser

Added tmux session chooser. Aliased to `tm`, calling `tmux_attach_start`
(the original tm script).

When called without arguments displays a fzf list of currently running
tmux sessions, with a preview to their respective open panes. A session
can be chosen in fzf which tmux will attach itself to. When creating a
query in fzf which does not have a valid target and confirming, tmux
will automatically create that session and attach itself to it.

When called with an argument, tmux will attach itself or create a
session of the same name. If called with the name of a session file, as
before, tmux will automatically execute that session file and attach
itself to it.

tmux/.config/sh/alias.d/tmux.sh

@@ -0,0 +1,49 @@
+#!/usr/bin/env sh
+
+exist() { type "$1" >/dev/null 2>&1; }
+
+# show a list of running tmux sessions
+alias tl='tmux list-sessions -F "#{session_name}" 2>/dev/null'
+
+# fzf
+if exist fzf; then
+  # fzf select a tmux session to connect to, with pane preview
+  alias tm='fzf_tmux'
+fi
+
+fzf_tmux() {
+  if [ -z "$1" ]; then
+    result=$(
+      tl | fzf \
+        --layout=reverse \
+        --height=50% \
+        --border \
+        --prompt="Session> " \
+        --preview="tmux_pane_tree {}" \
+        --preview-window=right:99% \
+        --print-query
+    )
+    case "$?" in
+    0)
+      # found a session, attaching
+      tmux_attach_start "$(echo "$result" | tail --lines=1)"
+      ;;
+    1)
+      # did not find a session, creating
+      result=$(echo "$result" | head --lines=1)
+      # if . was only thing entered, create one for current dir
+      if [ "$result" = "." ]; then
+        tmux_attach_start
+        # create for query name
+      else
+        tmux_attach_start "$result"
+      fi
+      ;;
+    esac
+  else
+    tmux_attach_start "$1"
+  fi
+}
+
+unset choice
+unset -f exist