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/heuristics.go

@@ -0,0 +1,35 @@
+package data
+
+import "gopkg.in/src-d/enry.v1/data/rule"
+
+// Heuristics implements a rule-based content matching engine.
+
+// Heuristics is a number of sequntially applied rule.Heuristic where a
+// matching one disambiguages language(s) for a single file extension.
+type Heuristics []rule.Heuristic
+
+// Match returns languages identified by the matching rule of the heuristic.
+func (hs Heuristics) Match(data []byte) []string {
+	var matchedLangs []string
+	for _, heuristic := range hs {
+		if heuristic.Match(data) {
+			for _, langOrAlias := range heuristic.Languages() {
+				lang, ok := LanguageByAlias(langOrAlias)
+				if !ok { // should never happen
+					// reaching here means language name/alias in heuristics.yml
+					// is not consistent with languages.yml
+					// but we do not surface any such error at the API
+					continue
+				}
+				matchedLangs = append(matchedLangs, lang)
+			}
+			break
+		}
+	}
+	return matchedLangs
+}
+
+// matchString is a convenience used only in tests.
+func (hs *Heuristics) matchString(data string) []string {
+	return hs.Match([]byte(data))
+}