Restore support for appending FORGIT_INSTALL_DIR to path (#308)

It seems somewhere along the line FORGIT_INSTALL_DIR stopped getting exported. According to the README you can append it to your path to get access to git forgit ... commands however this seems to no longer work - this is an attempt to restore this functionality.

conf.d/forgit.plugin.fish

@@ -1,9 +1,11 @@
 # MIT (c) Chris Apple
 
 set INSTALL_DIR (dirname (dirname (status -f)))
-set FORGIT "$INSTALL_DIR/conf.d/bin/git-forgit"
+set -x FORGIT_INSTALL_DIR "$INSTALL_DIR/conf.d"
+set -x FORGIT "$FORGIT_INSTALL_DIR/bin/git-forgit"
 if [ ! -e "$FORGIT" ]
-    set FORGIT "$INSTALL_DIR/vendor_conf.d/bin/git-forgit"
+    set -x FORGIT_INSTALL_DIR "$INSTALL_DIR/vendor_conf.d"
+    set -x FORGIT "$FORGIT_INSTALL_DIR/bin/git-forgit"
 end
 
 function forgit::warn

forgit.plugin.zsh

@@ -130,6 +130,8 @@ forgit::ignore::clean() {
     "$FORGIT" ignore_clean "$@"
 }
 
+export FORGIT_INSTALL_DIR=$INSTALL_DIR
+
 # register aliases
 # shellcheck disable=SC2139
 if [[ -z "$FORGIT_NO_ALIASES" ]]; then