cli: add --no-context-separator flag

--context-separator='' still adds a new line separator, which could
still potentially be useful. So we add a new `--no-context-separator`
flag that completely disables context separators even when the -A/-B/-C
context flags are used.

Closes #1390

complete/_rg

@@ -277,6 +277,7 @@ _rg() {
     ))'
     '*--colors=[specify color and style settings]: :->colorspec'
     '--context-separator=[specify string used to separate non-continuous context lines in output]:separator'
+    $no"--no-context-separator[don't print context separators]"
     '--debug[show debug messages]'
     '--dfa-size-limit=[specify upper size limit of generated DFA]:DFA size (bytes)'
     "(1 stats)--files[show each file that would be searched (but don't search)]"