Add install hook with 2.2.x support instructions.

Fixes #124

hooks/install.fish

@@ -0,0 +1,19 @@
+set -l required_version 2.3.0
+
+set -l installed_version 1
+if set -q FISH_VERSION
+  set installed_version $FISH_VERSION
+else if set -q version
+  set installed_version $version
+end
+
+set -l latest_version (echo -e "$required_version\\n$installed_version" | command sort -r -n -t '.' -k 1,1 -k 2,2 -k 3,3 -k 4,4 | command head -n 1)
+
+if [ "$installed_version" != "$latest_version" ]
+  set_color red ^/dev/null
+  echo "Fish $required_version or greater is required for bobthefish."
+  echo
+  echo "To use bobthefish with Fish $installed_version, checkout the `support/fish-2.2.x` branch"
+  echo "in $OMF_PATH/themes/bobthefish/"
+  set color normal ^/dev/null
+end