Function parse_issue_frontmatter 

pub fn parse_issue_frontmatter(issue: &Issue, target_field: &str) -> ParsedIssue

Parse YAML frontmatter from an issue body, extracting the target repo field.

Frontmatter is delimited by --- on its own line at the very start of the body. Only the field named by target_field is extracted; all other frontmatter is ignored. The returned body_without_frontmatter has the frontmatter block stripped.