fluent 0.3.0 (August 3, 2018)

• Add support for custom functions in MessageContext. (#50)
• Switch error handling to annotate-snippets crate.
• Separate fluent and fluent-syntax crates.
• Handle cyclic references. (#55)
• Switch parser binary to use clap.
• Switch plural rules handling to intl_pluralrules. (#56)
• Add FluentValue::as_number
• Move IntlPluralRules initialization into MessageContext::new
• General cleanups in line with cargo fmt and cargo clippy