Add path truncate.

I've add some truncate support of the path.

`g:EchoFuncPathMappingEnabled` can be:
+ 2, map `/user/local/include/foo/bar/baz` to `foo/bar/baz`
+ 3, map `/user/local/include/foo/bar/baz` to `local:foo/bar/baz`
+ 4, map `/user/local/include/foo/bar/baz` to `/u/l/i/f/b/baz` (like gvim guitablabel's default)
+ 5, same as 4
+ 6, map `/user/local/include/foo/bar/baz` to ~~foo/b/baz~~ `f/b/baz`
+ 7, map `/user/local/include/foo/bar/baz` to ~~local:foo/b/baz~~ `local:f/b/baz`