Add addTag / addExtra to the builder API

[tlvenn]

Pretty much anything that is needed to keep a fluent API is already done but if I need to add a tag or a meta, it breaks.

The builder should expose 2 methods such as:

addTag(String key, String value)
addMeta(String key, String value)

[joshdholtz]

Thanks for the suggestion! I can work on adding this in.

[tlvenn]

@joshdholtz Happy to provide a PR if you want