lexer: allow variables to be any valid CSS identifier

Previously, variables were required to be a "name" token--alphanumeric,
`-`, or `_` only. Allow variable names to be any valid identifier for
consistency with mixin/function names and Ruby Sass.

Satisfies tests introduced by sass/sass-spec#94.

Fixes #553.
Merge pull request #561 from benesch/variable-identifiers

lexer: allow variables to be any valid CSS identifier