Add --path-separator flag.

This flag permits setting the path separator used for all file paths
printed by ripgrep in normal operation.

Fixes #275

doc/rg.1

@@ -186,7 +186,7 @@ One byte is equal to one column.
 .RS
 .RE
 .TP
-.B \-\-context\-separator \f[I]ARG\f[]
+.B \-\-context\-separator \f[I]SEPARATOR\f[]
 The string to use when separating non\-continuous context lines.
 Escape sequences may be used.
 [default: \-\-]
@@ -328,6 +328,16 @@ and \-\-files.
 .RS
 .RE
 .TP
+.B \-\-path\-separator \f[I]SEPARATOR\f[]
+The path separator to use when printing file paths.
+This defaults to your platform\[aq]s path separator, which is / on Unix
+and \\ on Windows.
+This flag is intended for overriding the default when the environment
+demands it (e.g., cygwin).
+A path separator is limited to a single byte.
+.RS
+.RE
+.TP
 .B \-p, \-\-pretty
 Alias for \-\-color=always \-\-heading \-n.
 .RS