/loaders: add index of important loaders

[simon04]

• Move all loader documentation to /concepts/loaders
• List various loaders at /loaders, partially based on https://github.com/webpack/webpack/blob/2ed6448/README.md#loaders

[bebraw]

I wonder what to do with those license links. I guess we should resolve them to the packages somehow as now it's trying to look up against the site itself.

[simon04]

The idea of those license links/checks is to make sure that the included content is under a compatible license?

[bebraw]

The links are broken in npm too due to the way they have been written. I guess the best fix would be to fix the links at each package for this reason rather than work around here.

[simon04]

Hopefully those PRs will be merged quickly 😄

[bebraw]

@simon04 Merged the PRs.

[simon04]

Thank you! Now the Travis build succeeds.