Support complex enum values and ordering

[jeremy-ebler-vineti]

• Quote enum values (to allow values with spaces, quotes, etc)
• Preserve enum value ordering
• Enhance add_enum_value to allow inserting values before or after other values
• Dump to db/schema.rb with a one value per line format, for readable diffs

#quote comes from ActiveRecord::ConnectionAdapters::Quoting

You can ORDER BY an enum column, so being able to control the order can be quite useful.