Do not fail on unexisting GitHub/GitLab template variables

[orhun]

Is there an existing issue or pull request for this?

• I have searched the existing issues and pull requests

Feature description

When the github or gitlab feature isn't enabled and any remote-related template variable such as github.contributors is being used, git-cliff fails with the following error:

$ cargo run --no-default-features -- -c examples/github.toml

 ERROR git_cliff > Template render error:
Variable `github.contributors` not found in context while rendering 'template'

Desired solution

We should warn the user with a message like:

WARN: You seem to be using a GitHub-related variable but GitHub feature is not enabled.

Alternatives considered

None.

Additional context

I think we can add the RemoteReleaseMetadata to the Release struct even though the GitHub feature isn't enabled and warn the user instead (via checking the used variables via contains_variable method).