Fix cli parsing

[andrevidela]

The documentation reads

--config  Preamble configuration file in Dhall format.

However, writing --config *config file* would break with an obscure error message.

This is because the filename would be passed as argument directly to the Dhall parser, rather than being read from the filesystem and be given to the dhall parser.

The reason it worked until now is because if you write ./filepath, dhall knows to interpret it as a file to read from the filesystem and will perform the file reading for us. If you write filepath the dhall parser will fail assuming that it is an unbound variable. This PR fixes this behaviour by reading the file before passing its content to the parser.

I also fixed some warnings due to conflicting fixity declarations.