Rewrote substition

It now acts more like sed, but is a little more forgiving.
Added the i flag for case insensitivity, which can be used with the global flag.

Slashes can now be properly escaped and replaced, and you can use any character as the delimiter.

Everything below is valid:

:s/a/b/
:s/a/b
:s/a/b/gi
:s/a/b/g
:s/\//\\
:s/a/\\
:s_a_b
etc..