functions: add vimscratch

use `vimscratch` to open a scratch vim buffer.

If run in a pipe, it uses `stdin` as input:

```bash
<command> | vimscratch
```

functions/vim.zsh

@@ -0,0 +1,12 @@
+# open vim with a scratch window that can be discarded on exit.
+vimscratch() {
+    local args
+    # if running in a pipe, use stdin (-) as arg.
+    # -t checks if the given FD is a terminal
+    if [ -t 0 ] ; then
+        args="$@"
+    else
+        args="-"
+    fi
+    vim -c "set buftype=nofile" "$args"
+}

themes/brethil.zsh-theme

@@ -41,7 +41,7 @@ function job_prompt() {
 
 function shlvl_prompt() {
   local SHLVL_PROMPT
-  if [[ -n "$BYOBU_PREFIX" ]]; then
+  if [[ -n "$TMUX" ]]; then
     SHLVL_PROMPT="%(3L.%K{161}%F{255}%B $((SHLVL-1)) %f%b%k.)" # Shows SHLVL on a magenta background if SHLVL > 1
   else
     SHLVL_PROMPT="%(2L.%K{161}%F{255}%B %L %f%b%k.)" # Shows SHLVL on a magenta background if SHLVL > 1