Added an option 'o' for setting custom surround

It is sometimes nice to be able to just type in the expression rather
then prepare it ahead of the time.

It is sometimes nice to be able to just type in the expression rather
then prepare it ahead of the time.

Say if you want to refactor the code for which you do not have support
of the system. Or say you are editing github markdown and you want to
encapsulate the code in ``` tags. With this you can simply do thie
following

```
class test
    def initialise

end

Select the text then:
gSo```ruby\r```