Allow different program for R console

[randy3k]

Right now, it is assumed that the default R console is used in the terminal. It has been brought up by
@joshualeond that an alternative is rtichoke. We should make it possible to specify a different program to use in the terminal.

That means we need to change getRpath() to another function eg getRconsole() which reads settings r.console.mac, r.console.windows and r.console.linux. If they are not specified, we could use r.term.xxx as fallbacks.

[Ikuyadeu]

@randy3k Thank you for information!
I added r.rpath.lsp to use R.exe for language server.