Support for raw links and URL encoding

[SvenAelterman]

When adding a "Deploy to Azure" button in a README.md file, the reference that's used needs to be to an ARM template (JSON) file in the repository, but the raw file needs to be referenced. In addition, because the URL to the ARM template is specified as a query string parameter, the URL needs to be encoded.

This leads to hardcoding the raw URL, which includes the branch name, etc.

It would be nice if gitdown had two additional options:

1. Generate a link to a raw file in the current repo, current branch.
2. URL encode the link.

For the docs on creating a "Deploy to Azure" button see https://docs.microsoft.com/en-us/azure/azure-resource-manager/templates/deploy-to-azure-button.