Add template support to issue create, pr create

If multiple templates are found, the user is prompted to select one.

The templates are searched for, in order of preference:

- issues:
  1. `.github/ISSUE_TEMPLATE/*.md`
  2. `.github/ISSUE_TEMPLATE.md`
  3. `ISSUE_TEMPLATE/*.md`
  4. `ISSUE_TEMPLATE.md`
  5. `docs/ISSUE_TEMPLATE/*.md`
  6. `docs/ISSUE_TEMPLATE.md`

- pull requests:
  1. `.github/PULL_REQUEST_TEMPLATE/*.md`
  2. `.github/PULL_REQUEST_TEMPLATE.md`
  3. `PULL_REQUEST_TEMPLATE/*.md`
  4. `PULL_REQUEST_TEMPLATE.md`
  5. `docs/PULL_REQUEST_TEMPLATE/*.md`
  6. `docs/PULL_REQUEST_TEMPLATE.md`

The filename matches are case-insensitive.

pkg/githubtemplate/github_template.go

@@ -0,0 +1,99 @@
+package githubtemplate
+
+import (
+	"io/ioutil"
+	"path"
+	"regexp"
+	"sort"
+	"strings"
+
+	"gopkg.in/yaml.v3"
+)
+
+// Find returns the list of template file paths
+func Find(rootDir string, name string) []string {
+	results := []string{}
+
+	// https://help.github.com/en/github/building-a-strong-community/creating-a-pull-request-template-for-your-repository
+	candidateDirs := []string{
+		path.Join(rootDir, ".github"),
+		rootDir,
+		path.Join(rootDir, "docs"),
+	}
+
+	for _, dir := range candidateDirs {
+		files, err := ioutil.ReadDir(dir)
+		if err != nil {
+			continue
+		}
+
+		// detect multiple templates in a subdirectory
+		for _, file := range files {
+			if strings.EqualFold(file.Name(), name) && file.IsDir() {
+				templates, err := ioutil.ReadDir(path.Join(dir, file.Name()))
+				if err != nil {
+					break
+				}
+				for _, tf := range templates {
+					if strings.HasSuffix(tf.Name(), ".md") {
+						results = append(results, path.Join(dir, file.Name(), tf.Name()))
+					}
+				}
+				if len(results) > 0 {
+					goto done
+				}
+				break
+			}
+		}
+
+		// detect a single template file
+		for _, file := range files {
+			if strings.EqualFold(file.Name(), name+".md") {
+				results = append(results, path.Join(dir, file.Name()))
+				break
+			}
+		}
+		if len(results) > 0 {
+			goto done
+		}
+	}
+
+done:
+	sort.Sort(sort.StringSlice(results))
+	return results
+}
+
+// ExtractName returns the name of the template from YAML front-matter
+func ExtractName(filePath string) string {
+	contents, err := ioutil.ReadFile(filePath)
+	if err == nil && detectFrontmatter(contents)[0] == 0 {
+		templateData := struct {
+			Name string
+		}{}
+		if err := yaml.Unmarshal(contents, &templateData); err == nil && templateData.Name != "" {
+			return templateData.Name
+		}
+	}
+	return path.Base(filePath)
+}
+
+// ExtractContents returns the template contents without the YAML front-matter
+func ExtractContents(filePath string) []byte {
+	contents, err := ioutil.ReadFile(filePath)
+	if err != nil {
+		return []byte{}
+	}
+	if frontmatterBoundaries := detectFrontmatter(contents); frontmatterBoundaries[0] == 0 {
+		return contents[frontmatterBoundaries[1]:]
+	}
+	return contents
+}
+
+var yamlPattern = regexp.MustCompile(`(?m)^---\r?\n(\s*\r?\n)?`)
+
+func detectFrontmatter(c []byte) []int {
+	if matches := yamlPattern.FindAllIndex(c, 2); len(matches) > 1 {
+		return []int{matches[0][0], matches[1][1]}
+	}
+	return []int{-1, -1}
+}