ripgrep: add --line-buffered and --block-buffered

These flags provide granular control over ripgrep's buffering strategy.
The --line-buffered flag can be genuinely useful in certain types of shell
pipelines. The --block-buffered flag has a murkier use case, but we add it
for completeness.

complete/_rg

@@ -44,6 +44,12 @@ _rg() {
     '(: * -)'{-h,--help}'[display help information]'
     '(: * -)'{-V,--version}'[display version information]'
 
+    + '(buffered)' # buffering options
+    '--line-buffered[force line buffering]'
+    $no"--no-line-buffered[don't force line buffering]"
+    '--block-buffered[force block buffering]'
+    $no"--no-block-buffered[don't force block buffering]"
+
     + '(case)' # Case-sensitivity options
     {-i,--ignore-case}'[search case-insensitively]'
     {-s,--case-sensitive}'[search case-sensitively]'