Make pom dependency optional (but default)

[z33ky]

A downside is that the add_barcode example is not compiled for the nom_parser feature. I don't know how to express that, or if it is even possible. It seems like a very minor drawback though.

[J-F-Liu]

Is there a way to reduce the number of #[cfg] attributes?

[z33ky]

I could move the "auxiliary parsing" methods to a parser module (likewise for the tests), so only the #[cfg]s in lib.rs are required to enable them.
Alternatively a no-op dummy parser could be created, but I think that's a worse idea. I'll try my first suggestion.

[z33ky]

From 25 down to 75 added #[cfg]s. I skipped splitting up the tests, since that would require an required_features-entry in the Cargo.toml, which would then, like the add_barcode example only be available for pom_parser, but not nom_parser.