v0.3.0 — serde support

• Formatting InputArg::Stdin or OutputArg::Stdout with {:#} will now produce "<stdin>" or "<stdout>", respectively
• Adjusted the trait bounds on the from_arg() methods and the From implementations. Pre-existing code that uses these features will still work, but now some more types are accepted, and there should be no more needless copying of data.
• Added "serde" feature to enabling serializing & deserializing with serde