support different dist files

[rsgalloway]

currently distman looks for a dist.json file in the specified directory and dists all the targets.

technically it is possible to specify multiple targets in the dist file using wildcards, e.g.:

$ distman -t build-*

but it requires organizing targets using naming conventions, so it might be good to support multiple dist files as well.

contains build targets:

$ distman dist.build.json

contains dev targets:

$ distman dist.dev.json