Replace external tools with fish builtins

Fix issue #22
Fix issue #21

Replace use of external tools (grep and perl) with fish builtin string.

conf.d/__fasd_run.fish

@@ -1,10 +1,10 @@
 function __fasd_expand_vars -d "Expands only the first occurance of a variable in the passed string without evaluating the string"
-  set -lx vars (echo -n $argv | grep -oP '(?!\\\\)\$\K([A-z_][A-z0-9_]*?)([^A-z0-9_]|\b|\n)' | perl -pe 's/(.+?)(?:[^A-z0-9_]|\b)$/\1\n/' | sort -u)
+  set -lx vars (string match -a -r '(?!\\\\)\$\K([A-z_][A-z0-9_]*?)(?:[^A-z0-9_]|\b|\n)' "$argv" | string replace -r '(.+?)(?:[^A-z0-9_]|\b)$' '$1' | sort -u)
   for var in $vars
     # Only replace if the variable is defined
     if set -q $var
       # Replacing the variable once is enough
-      set argv (string replace -r '([^\\\\]|\b)\$'"$var" '$1'"$$var" "$argv") 
+      set argv (string replace -r '([^\\\\]|\b)\$'"$var" '${1}'"$$var" "$argv")
     end
   end
   # The following pipe does the same thing as fasd --sanitize