Rethink the autoprefixer commandline option

[gustavnikolaj]

In #37 @papandreou made the point that the --autoprefixer commandline option is problematic because of it's boolean/string behaviour.

Additionally it is inconsistent with the corresponding flag in assetgraph-builder.

The last comment in the thread outlined these options to solve the problem at hand:

1. doing as assetgraph-builder and require the user to supply the configuration
2. adding a --autoprefixerOptions parameter to go with the flag.
3. require configuration (as 1) but allow the user to pass the value defaults

[gustavnikolaj]

I think the best option is to get in line with assetgraph-builder.