Better encoding, currentColor support and identifier escaping

[zlondrej]

This is batch of changes that was necessary for my SVG sprite tool.

• currentColor support:
  • I wanted sprite to be able to change color by external color attribute. (https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/color)
• Identifier escaping
  • I wanted to use . character as part of identifier. This is perfectly valid in HTML/XML. As such, CSS supports such identifier as well, the special character just have to be escaped.
• Unicode encoding
  • Default "encoding-less" ByteString approach doesn't work properly (loading and storing) when SVG contains some multi-byte characters. This specifically applies to CSS escaping for identifiers, which would escape the multi-byte characters as separate characters instead of single one when ByteString is used instead of String or Text.
• Flag parsing in path's arc
  • Some SVG optimizers can produce output like this a1.3 1.3 0 01-1.3-1.3 (yes, it's valid). You can see 01 in the output which no a single number, but two flags. Current path parser failed on such path segment.