Some AllowAttrs.Matching regexp are not anchored

[powerman]

It looks like this was already fixed in #3. Here are some examples, but my point is, all regexps should be reviewed and anchored unless there is a really good reason to not do this:

func (p *Policy) AllowStandardAttributes() {
    p.AllowAttrs(
        "lang",
    ).Matching(regexp.MustCompile(`[a-zA-Z]{2,20}`)).Globally()
    p.AllowAttrs("id").Matching(
        regexp.MustCompile(`[a-zA-Z0-9\:\-_\.]+`),
    ).Globally()

func (p *Policy) AllowTables() {
    p.AllowAttrs(
        "scope",
    ).Matching(
        regexp.MustCompile(`(?i)(?:row|col)(?:group)?`),
    ).OnElements("td", "th")
    p.AllowAttrs("nowrap").Matching(
        regexp.MustCompile(`(?i)|nowrap`),
    ).OnElements("td", "th")

Also it's probably good idea to fix examples in README in same way.