Read config filename from workflow rather than assuming the default location

[jsbrittain]

The Builder module (builder_web.py) currently reads the configuration file for a workflow from the default config/config.yaml file. Instead, parse the workflow file for the configfile tag and source from there.

[jsbrittain]

This issue is simplified by #89 as configurations are only loaded as needed, so the Snakefile can be loaded first, checked for config filenames and then used for config loading. Note that these filenames may need to propogate into the builder system, or be redetermined there.