Add --files-with-matches flag.

Closes #26.

Acts like --count but emits only the paths of files with matches,
suitable for piping to xargs. Both mmap and no-mmap searches terminate
after the first match is found. Documentation updated and tests added.

doc/rg.1

@@ -35,6 +35,11 @@ Only show count of line matches for each file.
 .RS
 .RE
 .TP
+.B \-l, \-\-files\-with\-matches
+Only show path of each file with matches.
+.RS
+.RE
+.TP
 .B \-\-color \f[I]WHEN\f[]
 Whether to use coloring in match.
 Valid values are never, always or auto.

doc/rg.1.md

@@ -29,6 +29,9 @@ the raw speed of grep.
 -c, --count
 : Only show count of line matches for each file.
 
+-l, --files-with-matches
+: Only show path of each file with matches.
+
 --color *WHEN*
 : Whether to use coloring in match. Valid values are never, always or auto.
   [default: auto]