cargo add: use predominant string style

[flying-sheep]

I use single-quoted literal strings for my version specs.

cargo add should recognize and respect that

[stiiifff]

I’m also interested in getting that one fixed. Tried to have a look at where in toml_edit the issue comes from but haven’t figured it out. If I get some pointers I’m willing to give it a try.

[ordian]

toml_edit is just a toml parser. Detecting a predominant style is arguably a higher level thing, so IMO it should be done in cargo-edit. Feel free to join https://gitter.im/toml_edit/Lobby to continue the conversation.

[epage]

When merging cargo add upstream., we made the decision to have a canonical style rather than trying to match everyone's individual style