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_test.go

@@ -0,0 +1,61 @@
+package data
+
+import (
+	"regexp"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"gopkg.in/src-d/enry.v1/data/rule"
+)
+
+var testContentHeuristics = map[string]*Heuristics{
+	".md": &Heuristics{ // final pattern for parsed YAML rule
+		rule.Or(
+			rule.MatchingLanguages("Markdown"),
+			regexp.MustCompile(`(^[-A-Za-z0-9=#!\*\[|>])|<\/ | \A\z`),
+		),
+		rule.Or(
+			rule.MatchingLanguages("GCC Machine Description"),
+			regexp.MustCompile(`^(;;|\(define_)`),
+		),
+		rule.Always(
+			rule.MatchingLanguages("Markdown"),
+		),
+	},
+	".ms": &Heuristics{
+		// Order defines precedence: And, Or, Not, Named, Always
+		rule.And(
+			rule.MatchingLanguages("Unix Assembly"),
+			rule.Not(rule.MatchingLanguages(""), regexp.MustCompile(`/\*`)),
+			rule.Or(
+				rule.MatchingLanguages(""),
+				regexp.MustCompile(`^\s*\.(?:include\s|globa?l\s|[A-Za-z][_A-Za-z0-9]*:)`),
+			),
+		),
+		rule.Or(
+			rule.MatchingLanguages("Roff"),
+			regexp.MustCompile(`^[.''][A-Za-z]{2}(\s|$)`),
+		),
+		rule.Always(
+			rule.MatchingLanguages("MAXScript"),
+		),
+	},
+}
+
+func TestContentHeuristic_MatchingAlways(t *testing.T) {
+	lang := testContentHeuristics[".md"].matchString("")
+	assert.Equal(t, []string{"Markdown"}, lang)
+
+	lang = testContentHeuristics[".ms"].matchString("")
+	assert.Equal(t, []string{"MAXScript"}, lang)
+}
+
+func TestContentHeuristic_MatchingAnd(t *testing.T) {
+	lang := testContentHeuristics[".md"].matchString(";;")
+	assert.Equal(t, []string{"GCC Machine Description"}, lang)
+}
+
+func TestContentHeuristic_MatchingOr(t *testing.T) {
+	lang := testContentHeuristics[".ms"].matchString("	.include \"math.s\"")
+	assert.Equal(t, []string{"Unix Assembly"}, lang)
+}