Improve conda activate

1) activate becomes fast by simply parsing a text file
2) activate is now also able to resolve environment without name
(specifically those with prefix)

_conda

@@ -128,7 +128,7 @@ local -A opt_args
 
 __conda_envs(){
     local -a envs
-    envs=($( conda info -e | sed "1,2d" | cut -f1 -d' '))
+    envs=($( cat $HOME/.conda/environments.txt | cut -f1 -d' '))
     _describe -t envs 'conda environments' envs
 }