Adding custom attributes, IO.Stream support, formatting fixes and more extensions

[dzaba1]

• Methods for adding custom attributes to nodes, edges and sub-graphs
• Methods for adding custom, raw lines to graphs and sub-graphs
• Fixed the issue with indented formatting sub-graphs attributes
• Compile methods now can use IO.Stream
• Fluent API
• Public API shouldn't be broken

Let me know if it's ok. I can also update the README

[vfrz]

Thanks again, lgtm. I'll update the nuget package asap!

[vfrz]

This is now available on NuGet as version 2.7.0