Add fetch-tiles CLI command

[brendancol]

integrate the tile-fetch scripts you prepared for inclusion in mapshader CLI

[rencesar]

We currently have:

• A entrypoint
• It will call the function in main.py

What We need to do:

• Accept multiple commands from mapshader not only pyct
• Make easy to add new commands
• Test the new Fetch tiles
• Implement Fetch tiles

[giancastro]

@rencesar, a commands module will be created on PR #65 to allow adding new commands, we can use pyct to create custom commands.