Introduce 'from' parameter in restx build generatecommand

[fcamblor]

Imported from restx/restx#172

It would be nice to improve the restx build generate <type> command into something like : restx build generate <dest_type> [from <src_type>]

Assuming from md would be implicit if not provided.

Thus we would be able to generate every dependencies descriptor types from every dependencies descriptor types.
For instance : restx build generate md from pom

It would imply to :

• Provide missing pom/ivy writers from md reader
• Provide missing md writers from pom/ivy readers

[fcamblor]

Quoting @xhanin :

Note that we already have the pom/md parsers and the md/pom/ivy writers.

So we are able to convert from pom/md to md/pom/ivy, we just need to support it in the command.