Add converter_templates config parameter

[guvra]

This PR adds a config parameter named converter_templates (to be confirmed).

It allows to declare reusable converter templates:

converter_templates:
    acmeEmail:
        converter: 'randomizeEmail'
        unique: true
        parameters:
            domains: ['acme.com']

tables:
    table1:
        converters:
            email:
                converter: 'acmeEmail'
    table2:
        converters:
            email:
                converter: 'acmeEmail'
                parameters:
                    min_length: 10

It also works for converter parameters, for example:

tables:
    table1:
        converters:
            email:
                converter: 'chain'
                parameters:
                    - converter: 'acmeEmail'