Add doc and symbol location support to vim plugin

`--doc` is supported with the `DCDdoc` command. I wanted to use the `K` key, but sadly it has to run an external program with the identifier under the cursor - I can't feed the current buffer text and the byte position to it...

`--symbolLocation` is supported with the `DCDsymbolLocation` command. It jumps to the declaration if DCD can find one.

I've also fixed [a bug where "free" symbols(=not after dot and not parenthesis) couldn't be autocompleted](http://forum.dlang.org/thread/utnxjyetwcpnpegqyrdd@forum.dlang.org#post-szzszkyivairvkgwlqtl:40forum.dlang.org).