add client.kak providing 'new' command for opening a new client

Autodetect tmux, use tmux if we are in a tmux session, else fallback to
urxvt.

README.asciidoc

@@ -454,3 +454,9 @@ Existing commands files are:
  * *rc/ctags.kak*: provides the +tag+ command to jump on a tag definition using
      exuberant ctags files, this script requires the *readtags* binary, available
      in the exuberant ctags package but not installed by default.
+ * *rc/client.kak*: provides the +new+ command to launch a new client on the current
+     session, if tmux is detected, launch the client in a new tmux split, else
+     launch in a new terminal emulator.
+
+Certain command files defines options, such as grepcmd (for :grep) or
+termcmd (for :new).

src/Makefile

@@ -23,6 +23,6 @@ XDG_CONFIG_HOME ?= $(HOME)/.config
 
 userconfig:
 	mkdir -p $(XDG_CONFIG_HOME)/kak/autoload
-	ln -rs rc/{asciidoc,cpp,diff,git,grep,kakrc,make}.kak $(XDG_CONFIG_HOME)/kak/autoload/
+	ln -rs rc/{asciidoc,client,cpp,diff,git,grep,kakrc,make}.kak $(XDG_CONFIG_HOME)/kak/autoload/
 
 .PHONY: tags userconfig

src/rc/client.kak

@@ -0,0 +1,13 @@
+setg termcmd %sh{
+    if [[ -n "$TMUX" ]]; then
+        echo "'tmux split-window -h'"
+    else
+        echo "'urxvt -e sh -c'"
+    fi
+}
+
+def new -shell-params %{ nop %sh{
+    if (( $# != 0 )); then kakoune_params="-e '$@'"; fi
+    ${kak_opt_termcmd} "kak -c ${kak_socket#/tmp/kak-} ${kakoune_params}" < /dev/null >& /dev/null &
+    disown
+}}