Support custom user CREW_PREFIX

The `${VAR:-/some/path}` will use the path on the right side if the variable on the left side is null or empty (ie if the user didn't set it).

install.sh

@@ -3,7 +3,7 @@ OWNER="skycocker"
 REPO="chromebrew"
 BRANCH="master"
 URL="https://raw.githubusercontent.com/$OWNER/$REPO/$BRANCH"
-CREW_PREFIX=/usr/local
+CREW_PREFIX=${CREW_PREFIX:-/usr/local}
 CREW_LIB_PATH=$CREW_PREFIX/lib/crew/
 CREW_CONFIG_PATH=$CREW_PREFIX/etc/crew/
 CREW_BREW_DIR=$CREW_PREFIX/tmp/crew/