Let purebred-mailcap build the ProcessConfig

[romanofski]

The mailcap provides templates on how shell commands are executed. @frasertweedale suggested one function, e..g:

prepareProcess :: MailcapEntry -> Message s B.Bytestring -> ProcessConfig i o e

provided by the library returning a process config. That config can then be executed by purebred.

[romanofski]

See #9 as the alternate approach

[frasertweedale]

I think I want to reject this approach already. Making / cleaning up temporary files involves a lot of policy decisions that i think it's better to leave to the application.

This does not entirely preclude building a ProcessConfig, though. I just means that purebred-mailcap should not be responsible for creating temporary files, etc.

[frasertweedale]

Closing in favour of #9.