Add session -p <project> flag

This flag sets the session name to `<project>` where `<project>` is a
git repository to make working with multiple worktrees more streamlined.

session/_session

@@ -4,6 +4,8 @@ __session_sessions() {
   list() {
     for item in $HOME/.local/share/tmux/layouts/session-*; do
       item=${item#$HOME/.local/share/tmux/layouts/}
+      # session-project is parameterized by -p, not a session of its own
+      [ "$item" = session-project ] && continue
       echo ${item#session-}
     done
   }
@@ -12,6 +14,10 @@ __session_sessions() {
   _describe 'session' sessions
 }
 
+__session_projects() {
+  _files -W $HOME/Projects -/
+}
+
 __session_hosts() {
   list() {
     declare -A hosts
@@ -28,5 +34,7 @@ __session_hosts() {
 }
 
 _arguments \
+  '(- *)'{-h,--help}'[Show usage information]' \
+  '(- *)'{-p,--project}'[Create or attach to a session for a project in ~/Projects]:project:__session_projects' \
   ':session:__session_sessions' \
   ':host:__session_hosts'