| should be inside of (…)

It's possible to use `(t|true|T|True)` as `truthy_pattern` but still failing `true` as a truthy under my environment :-(

```
Apple LLVM version 8.0.0 (clang-800.0.42.1)
Target: x86_64-apple-darwin15.6.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
```

\# clang-5.0.0 works as expected even if does not modify it.

include/cxxopts.hpp

@@ -444,7 +444,7 @@ namespace cxxopts
       std::basic_regex<char> integer_pattern
         ("(-)?(0x)?([1-9a-zA-Z][0-9a-zA-Z]*)|((0x)?0)");
       std::basic_regex<char> truthy_pattern
-        ("t|true|T|True");
+        ("(t|T)(rue)?");
       std::basic_regex<char> falsy_pattern
         ("(f|false|F|False)?");
     }