Sync to linguist 7.2.0: heuristics.yml support (#189)

Sync \w Github Linguist v7.2.0

Includes new way of handling `heuristics.yml` and
all `./data/*` re-generated using Github Linguist [v7.2.0](https://github.com/github/linguist/releases/tag/v7.2.0)
release tag.

 - many new languages
 - better vendoring detection
 - update doc on update&known issues.

data/rule/rule.go

@@ -0,0 +1,109 @@
+// Package rule contains rule-based heuristic implementations.
+// It is used in the generated code in content.go for disambiguation of languages
+// with colliding extensions, based on regexps from Linguist data.
+package rule
+
+// Heuristic consist of (a number of) rules where each, if matches,
+// identifes content as belonging to a programming language(s).
+type Heuristic interface {
+	Matcher
+	Languages() []string
+}
+
+// Matcher checks if the data matches (number of) pattern.
+// Every heuristic rule below implements this interface.
+// A regexp.Regexp satisfies this interface and can be used instead.
+type Matcher interface {
+	Match(data []byte) bool
+}
+
+// languages struct incapsulate data common to every Matcher: all languages
+// that it identifies.
+type languages struct {
+	langs []string
+}
+
+// Languages returns all languages, identified by this Matcher.
+func (l languages) Languages() []string {
+	return l.langs
+}
+
+// MatchingLanguages is a helper to create new languages.
+func MatchingLanguages(langs ...string) languages {
+	return languages{langs}
+}
+
+// Implements a Heuristic.
+type or struct {
+	languages
+	pattern Matcher
+}
+
+// Or rule matches, if a single matching pattern exists.
+// It recives only one pattern as it relies on compile-time optimization that
+// represtes union with | inside a single regexp.
+func Or(l languages, r Matcher) Heuristic {
+	return or{l, r}
+}
+
+// Match implements rule.Matcher.
+func (r or) Match(data []byte) bool {
+	return r.pattern.Match(data)
+}
+
+// Implements a Heuristic.
+type and struct {
+	languages
+	patterns []Matcher
+}
+
+// And rule matches, if each of the patterns does match.
+func And(l languages, m ...Matcher) Heuristic {
+	return and{l, m}
+}
+
+// Match implements data.Matcher.
+func (r and) Match(data []byte) bool {
+	for _, p := range r.patterns {
+		if !p.Match(data) {
+			return false
+		}
+	}
+	return true
+}
+
+// Implements a Heuristic.
+type not struct {
+	languages
+	Patterns []Matcher
+}
+
+// Not rule matches if none of the patterns match.
+func Not(l languages, r ...Matcher) Heuristic {
+	return not{l, r}
+}
+
+// Match implements data.Matcher.
+func (r not) Match(data []byte) bool {
+	for _, p := range r.Patterns {
+		if p.Match(data) {
+			return false
+		}
+	}
+	return true
+}
+
+// Implements a Heuristic.
+type always struct {
+	languages
+}
+
+// Always rule always matches. Often is used as a default fallback.
+func Always(l languages) Heuristic {
+	return always{l}
+}
+
+// Match implements Matcher.
+func (r always) Match(data []byte) bool {
+	return true
+}