Add new -M/--max-columns option.

This permits setting the maximum line width with respect to the number
of bytes in a line. Omitted lines (whether part of a match, replacement
or context) are replaced with a message stating that the line was
elided.

Fixes #129

doc/rg.1.md

@@ -203,6 +203,10 @@ Project home page: https://github.com/BurntSushi/ripgrep
 -L, --follow
 : Follow symlinks.
 
+-M, --max-columns *NUM*
+: Don't print lines longer than this limit in bytes. Longer lines are omitted,
+  and only the number of matches in that line is printed.
+
 -m, --max-count *NUM*
 : Limit the number of matching lines per file searched to NUM.