Download CommonMark and CiceroMark models from their published version about markdown-transform

[jeromesimeon]

@dselman ? It might be a good idea to rename the markdown.cto model to commonmark.cto, see accordproject/models#51
and in the README in this project.

[hritikgupta]

Hi @jeromesimeon, I would like to work on this issue. Please assign it to me.
Appreciate if you could also give a headstart to fix the issue.

[jeromesimeon]

@hritikgupta I think the main idea for how to fix this issue is:

1. Add concerto-cli to the developers dependencies
2. Create a script which uses cicero get to download the source CTO models and then generates the two files listed above.
3. Call that script during the build process.

There will be some need for cleaning up the .gitignore make sure Travis is still happy, etc.

@mttrbrts @dselman does that make sense?

[jeromesimeon]

@hritikgupta I'm afraid I have to move this issue forward for the coming release, so I'm reassigning it to myself. Apologies if you had already started working on it.

[jeromesimeon]

Completed and merged in #89