Allow a user to validate multiple CSV files

[ldodds]

A user should be able to point to a list of CSV files and have them all submitted for validation.

[JeniT]

UI development:

• [x] Load from single CSV URL
• [x] Load from multiple URLs in text box
• [x] Load from datapackage URL
• [x] Load from data.gov.uk URL
• [x] Upload single file
• [ ] Upload zipfile
• [x] Upload datapackage (as zipfile?)
• [x] Upload multiple files

[pezholio]

This is all pretty much done now. The only thing remaining is theodi/csvlint#112, which needs merging