Add --smart-case.

It does what it says on the tin.

Closes #70.

doc/rg.1

@@ -238,6 +238,12 @@ Alias for \-\-color=always \-\-heading \-n.
 .RS
 .RE
 .TP
+.B \-S, \-\-smart\-case
+Search case insensitively if the pattern is all lowercase.
+Search case sensitively otherwise.
+.RS
+.RE
+.TP
 .B \-j, \-\-threads \f[I]ARG\f[]
 The number of threads to use.
 Defaults to the number of logical CPUs (capped at 6).

doc/rg.1.md

@@ -154,6 +154,10 @@ the raw speed of grep.
 -p, --pretty
 : Alias for --color=always --heading -n.
 
+-S, --smart-case
+: Search case insensitively if the pattern is all lowercase.
+  Search case sensitively otherwise.
+
 -j, --threads *ARG*
 : The number of threads to use. Defaults to the number of logical CPUs
   (capped at 6). [default: 0]