use miniwdl to parse WDL

[leepc12]

Move parameters (as comments) for Croo into WDL workflow's meta section. This meta section is optional and any string key / string value is allowed.

Example for ENCODE ATAC-Seq pipeline:

#CROO out_def https://storage.googleapis.com/encode-pipeline-output-definition/atac.croo.v4.json

workflow atac {
    meta {
        author: 'Jin wook Lee (leepc12@gmail.com) at ENCODE-DCC'
        description: 'ATAC-Seq/DNase-Seq pipeline'

        croo_out_def: 'https://storage.googleapis.com/encode-pipeline-output-definition/atac.croo.v4.json'
    }
}

miniwdl's WDL package has parse_document function, which checks syntax of WDL but does not check imports. Use this function to get access to workflow's meta section.

Croo will still be able to parse comment-based parameters but it will be deprecated soon.