Support `+clientserver` editors

[sstallion]

Editors compiled with the +clientserver option allows expressions to be sent remotely to an existing process. This would allow interactive vimwiki commands to open pages in a different window to leave the command-line free for other uses. It looks like the --remote-expr flag will be the most likely entry point, however there will be some complicating factors around detecting whether a server is running before execution as this flag does not seem to have the same behavior as the other --remote flags.