Much faster startup through lazy-loading, and much more!

Big release this time thanks to tons of fantastic contributions from @Enselic, this release was basically all him! The headline feature is much faster startup time due to lazy-loading at the syntax level.

• Lazy-load syntaxes to significantly improve startup time
• Remove ContextId::new() from public API to support lazy-loading of syntaxes. This is technically a breaking change but I have no idea why I made this API public and I'm pretty sure zero people have used it.
• Add ScopeRangeIterator
• Add CI check for Minimum Supported Rust Version. This is currently Rust 1.51.
• Make 'plist' dependency (used for loading themes) optional via new 'plist-load' feature
• Make looking up a syntax by extension use case-insensitive comparison
• Make from_dump_file() ~15% faster
• Blend alpha value on converting colors to ANSI color sequences
• Fix sample code in documentation to avoid double newlines
• Fix lots of build warnings and lints
• Add Criterion benchmarks for a whole syntect pipeline and for from_dump_file()
• Replace lazycell with once_cell to fix crash on lazy initialization