General improvements for --chdir

## New Usage

```
Usage: migrate [options] [command]

Options:

   -e, --env <name>   change the environment

Commands:

   down             migrate down
   up               migrate up (the default command)
   create [title]   create a new migration file with optional [title]
   copy [to, from]  copy migrations from one environment context to another

```

## Running Migrations with Environment Context

      $ migrate -e dev create
        create : migrations/dev/001.js

      $ migrate -e dev up
        up : migrations/dev/001.js
        migration : complete

      $ migrate -e dev down
        down : migrations/dev/001.js
        migration : complete

      $ migrate copy dev prod
        copying : migrations/dev/001.js to migrations/prod/001.js

      $ migrate -e prod up
        up : migrations/prod/001.js
        migration : complete

      $ migrate -e prod down
        down : migrations/prod/001.js
        migration : complete