Create and cache npm completion on first run

Signed-off-by: Marc Cornellà <marc.cornella@live.com>

plugins/npm/npm.plugin.zsh

-eval "$(npm completion 2>/dev/null)"
+__NPM_COMPLETION_FILE="${ZSH_CACHE_DIR}/npm_completion"
+
+if [[ ! -f $__NPM_COMPLETION_FILE ]]; then
+    npm completion >! $__NPM_COMPLETION_FILE || rm -f $__NPM_COMPLETION_FILE
+fi
+
+source $__NPM_COMPLETION_FILE
 
 # Install dependencies globally
 alias npmg="npm i -g "