~gbmor/rustwtxt

using custom error type instead of string
changed metadata parse section:
if both metadata heading and keyword aren't in the file, return err
further check for improperly formatted statuses
updated crate metadata
cut some unnecessary code
added docs.rs badge
changed doc comments around
updated integ test
documentation added for new methods
changed structs api
testing for panic in Twtxt::new()
Tweet/Twtxt structs tested
impl blocks for Twtxt/Tweet
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