Add -x/--line-regexp (#520)

add -x/--line-regexp flag

doc/convert-to-man

@@ -1,4 +1,5 @@
-#!/bin/sh
+#!/bin/sh -e
 
 pandoc -s -t man rg.1.md -o rg.1
-sed -i 's/\.TH.*/.TH "rg" "1"/g' rg.1
+sed -i.bak 's/\.TH.*/.TH "rg" "1"/g' rg.1
+rm -f rg.1.bak # BSD `sed` requires the creation of a back-up file