Support passing an alternate URL to pull as a second argument

Signed-off-by: Juan Cruz Viotti <jv@jviotti.com>

Author: jviotti

README.markdown

@@ -91,6 +91,15 @@ You can also pull a single dependency by specifying its name as the first argume
 ./vendor/vendorpull/pull depot_tools
 ```
 
+When pulling a single dependency, you can provide an explicit alternative
+location to pull from as a second argument. This is particularly handy if you
+want to pull from a local directory without the overhead of hitting an external
+git server:
+
+```sh
+./vendor/vendorpull/pull my-dependency ../projects/my-dependency
+```
+
 Updating
 --------
 

pull

@@ -91,7 +91,15 @@ then
     fail "Unknown dependency: $DEPENDENCY"
   fi
   NAME="$(echo "$LINE" | cut -d ' ' -f 1)"
-  URL="$(echo "$LINE" | cut -d ' ' -f 2)"
+
+  PULL_LOCATION="${2-}"
+  if [ -n "$PULL_LOCATION" ]
+  then
+    URL="$PULL_LOCATION"
+  else
+    URL="$(echo "$LINE" | cut -d ' ' -f 2)"
+  fi
+
   VERSION="$(echo "$LINE" | cut -d ' ' -f 3)"
   if [ -z "$NAME" ] || [ -z "$URL" ] || [ -z "$VERSION" ]
   then