Expose Parse and ToCss traits

[devongovett]

Part of #5.

This exposes the Parse and ToCss traits, which can be used to parse and serialize individual CSS rules, properties, and values rather than only entire stylesheets. I also added parse_string and to_css_string convenience functions to parse and serialize from a string, without manually constructing a parser/printer from scratch.

By extension, this means the Printer is also exposed. I've refactored it a little to accept an options struct and also to hide internal fields.

Example:

use parcel_css::{
  properties::Property,
  rules::CssRule,
  stylesheet::{ParserOptions, PrinterOptions},
  traits::{Parse, ToCss},
  values::color::CssColor,
};

// Parse and serialize an individual value
let color = CssColor::parse_string("#f0f")?;
color.to_css_string(PrinterOptions::default())?; // => "#f0f"

// Parse a property by name
let property = Property::parse_string("color", "#f0f", ParserOptions::default())?;
property.to_css_string(/* important */ false, PrinterOptions::default())?; // => "color: #f0f"

// Parse a single rule
let rule = CssRule::parse_string(".foo { color: red; }", ParserOptions::default())?;
rule.to_css_string(PrinterOptions::default())?; // => ".foo {\n  color: red;\n}"

cc. @lucacasonato

[lucacasonato]

Very nice!