Use clone trait instead of copy and make it optional

[LeonMatthes]

Hi, I wanted to use this library for a pet project I'm working on, and I noticed it only supports elements of the Copy trait, which seems pretty restrictive, so I allowed users to use it with the Clone trait, and made that trait optional too.

My workflow also used rustfmt to reformat the code, which kind of obfuscates the diff, sorry about that, I only made real changes to the impl blocks around from_example and resize.

Because this is introduces a backward compatible API change, I bumped the version number to 0.2.0

[nicholasbishop]

Thanks for the patch :) I went ahead and formatted the repo with cargo fmt to reduce the diff here, could you rebase your changes on that? Thanks

[LeonMatthes]

Alright, the diff had a few merge conflicts due to both of us using rust fmt, but Github helped me resolve that quickly. Everything's clean now.

Just FIY, I'll probably add another PR in a few minutes that adds implementations for serde Serialize and Deserialize traits once I read up on how to use cargo features.
I will try and keep the two PRs separate from each other in case you want to merge one, but not the other.