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.

git/git.go

@@ -151,6 +151,14 @@ func ReadBranchConfig(branch string) (cfg BranchConfig) {
 	return
 }
 
+// ToplevelDir returns the top-level directory path of the current repository
+func ToplevelDir() (string, error) {
+	showCmd := exec.Command("git", "rev-parse", "--show-toplevel")
+	output, err := utils.PrepareCmd(showCmd).Output()
+	return firstLine(output), err
+
+}
+
 func outputLines(output []byte) []string {
 	lines := strings.TrimSuffix(string(output), "\n")
 	return strings.Split(lines, "\n")