Support unix convention of using - for stdin.

[matyalatte]

You can use - to read code from stdin.

$ echo "int a = (int)1.0;" | cpplint-cpp -
Done processing -
-:0:  No copyright message found.  You should have a line: "Copyright [year] <Copyright Owner>"  [legal/copyright] [5]
-:1:  Using C-style cast.  Use static_cast<int>(...) instead  [readability/casting] [4]
Total errors found: 2