Update HCL parser to enable parsing of provider functions

[tpdownes]

Resolves #7.

In private code, I am using a provider function with this syntax:

provider::azurerm::parse_resource_id(yara.id)["resource_name"]

Before

$ ~/repos/terraform-diff/terraform-diff -range HEAD~...HEAD  ./my/path
failed getting dependencies for folder ./my/path: Error processing: Invalid 'for' expression: Extra characters after the end of the 'for' expression.

After

$ ~/repos/terraform-diff/terraform-diff -range HEAD~...HEAD  ./my/path
./my/path

[tpdownes]

@yannh PTAL. Thank you for this tool!

[tpdownes]

This enables me to fork the repo and publish my own image as a workaround to #7. Without this, the publish-branch step fails.

[ohookins]

Cheers @tpdownes !

[ohookins]

@yannh could we get this merged?