encoder / hcl.Marshal() support?

This isn't a bug -- but rather a support/feature request for a Marshal method that will take a map of configuration data and output a hcl file. Something like;

```go
func Marshal(data map[string]interface{}) ([]byte, error) {
hcl = []byte
for k, v in data {
...
}
return hcl
}
```

I'm working on a transmorgifying configuration tool that allows reading of arbitrary configuration data (json, yaml, toml, xml, and shellvars) and uses it to either
* render a template
* output as a unified format (e.g. as json or yml or hcl ! :) )

supporting hcl would be great, although I can only include it as input data and not output if the library is not bi-directional. Does it make sense to add something like this?

Thanks!

Start by reviewing the encoder API and the proposed hcl.Marshal() entry point, then determine how configuration maps should be represented in HCL. Done means an agreed Marshal interface that produces valid HCL output for the requested configuration data, with behavior and compatibility clarified.

Written by the indexing model from the issue text.