elm command options changed

```bash
$ elm --help
The Elm Compiler 0.13, (c) Evan Czaplicki 2011-2014

flags [OPTIONS] [FILES]
  Compile Elm programs to HTML, CSS, and JavaScript.

Common flags:
  -m --make              automatically compile dependencies.
     --set-runtime=FILE  Specify a custom location for Elm's runtime system.
  -g --get-runtime       Print the absolute path to the default Elm runtime.
     --bundle-runtime    Bundle the runtime with the generated html or js to
                         create a standalone file.
  -o --only-js           Compile only to JavaScript.
  -p --print-types       Print out inferred types of top-level definitions.
     --scripts=FILE      Load JavaScript files in generated HTML. Files will
                         be included in the given order.
  -n --no-prelude        Do not import Prelude by default, used only when
                         compiling standard libraries.
  -c --cache-dir=FILE    Directory for files cached to make builds faster.
                         Defaults to cache/ directory.
     --build-dir=FILE    Directory for generated HTML and JS files. Defaults
                         to build/ directory.
     --src-dir=FILE      Additional source directories besides project root.
                         Searched when using --make
  -h --help              Display help message
  -v --version           Print version information
     --numeric-version   Print just the version number
```
change elm command, -b no longer exists

Since elm-0.13, -b option is not supported, use --build-dir and
--cache-dir instead of -b and -c.
Merge branch 'master' of https://github.com/qzchenwl/elm.vim
Merge pull request #11 from qzchenwl/master

Change elm option to print types.