Change executable name to "darfmt". Fix #163.

README.md

 The dart_style package defines an automatic, opinionated formatter for Dart
 code. It replaces the whitespace in your program with what it deems to be the
 best formatting for it. Resulting code should following the [Dart style guide][]
 but, moreso, should look nice to most human readers, most of the time.
 
 It handles indentation, inline whitespace and (by far the most difficult),
 intelligent line wrapping. It has no problems with nested collections, function
 expressions, long argument lists, or otherwise tricky code.
 
-**The formatter is at a beta state right now. It does a good job on most code,
-and I'd love to have you try it and [report bugs][bugs], but its output may
-change over time.**
-
 ## Running it
 
 The package exposes a simple command-line wrapper around the core formatting
 library. The easiest way to invoke it is to [globally activate][] the package
 and let pub put its executable on your path:
 
     $ pub global activate dart_style
-    $ dartformat ...
+    $ dartfmt ...
 
 If you don't want `dartformat` on your path, you can run it explicitly:
 
     $ pub global activate dart_style --no-executables
     $ pub global run dart_style:format ...
 
 The formatter takes a list of paths, which can point to directories or files.
 If the path is a directory, it processes every `.dart` file in that directory
 or any of its subdirectories.
 
@@ skipping 45 matching lines @@
 If it does do so, this is a critical bug and we'll fix it quickly.
 
 The rules the formatter uses to determine the "best" way to split a line may
 change over time. We don't promise that code produced by the formatter today
 will be identical to the same code run through a later version of the formatter.
 We do hope that you'll like the output of the later version more.
 
 [bugs]: https://github.com/dart-lang/dart_style/issues
 [dart style guide]: https://www.dartlang.org/articles/style-guide/
 [globally activate]: https://www.dartlang.org/tools/pub/cmd/pub-global.html