Add support for Avro IDL

[mikaello]

Avro schemata can be defined in both JSON and Avro IDL. Currently avrodoc-plus only supports JSON, but it would be nice if we could support Avro IDL as well.

Avro IDL is a higher-level language for authoring Avro schemata, from the docs:

The aim of the Avro IDL language is to enable developers to author schemata in a way that feels more similar to common programming languages like Java, C++, or Python. Additionally, the Avro IDL language may feel more familiar for those users who have previously used the interface description languages (IDLs) in other frameworks like Thrift, Protocol Buffers, or CORBA.

[karstenspang]

Agree, that would be nice. Currently, I generate JSON files just for the purpose of Avrodoc. This works fine, except that the file name column on the overview page is useless, as there are no files in the source tree with those names.