Guess if .r is R or REBOL

.r is the default extension used widely for both, R and REBOL. This
patch attempts to guess if .r is a REBOL file by looking for the
following very common REBOL code fragments:

- `REBOL`
- `: func [`
- `make object! [`
- `context [`

If any of those is found in a .r file, it is identified as a REBOL file.
Otherwise the language for a .r file is R.

Signed-off-by: Andreas Bolka <a@bolka.at>

test/fixtures/hello-r.R

@@ -0,0 +1,4 @@
+hello <- function() {
+    print("hello, world!")
+}
+hello()