Animation name causes "known-properties" warning if name contains the underscore character (_)

[mattiacci]

Copy of gamtiq's issue in csslint: CSSLint/csslint#450

The following code causes the warning about unexpected property value:
Expected (none | <ident>) but found 'a-move_vert'

.move {
   animation-name: a-move_vert;
}

From what I can tell reading about CSS3 syntax, underscores are valid in identifiers.