WIP: Rewrite config with annotations and macros

[viktorerlingsson]

Work in progress / suggestion

WHAT is this pull request doing?

Rewrites config.cr with the use of annotations and macros to specify how each property can be set. Each property can have multiple annotations to be available to configure in multiple ways.

    @[CliOpt("-D DIRECTORY", "--data-dir=DIRECTORY", "Data directory")]
    @[IniOpt(section: "main")]
    @[EnvOpt("LAVINMQ_DATADIR")]
    property data_dir : String = "/var/lib/lavinmq"

@[CliOpt("-D DIRECTORY", "--data-dir=DIRECTORY", "Data directory")]
for options to be configured by command line

@[IniOpt(section: "main")]
for options to be configured by config file

@[EnvOpt("LAVINMQ_DATADIR")]
for options to be configured by env variables

HOW can this pull request be tested?

No specs yet, however, it should be possible to cover this pretty well with specs.