handle invalid front-matter errors (#243)

Use an object config to skip gray-matter's caching (see jonschlinkert/gray-matter#166)
closes #215

src/markdown.ts

@@ -422,7 +422,8 @@ function toParseCells(pieces: RenderPiece[]): CellPiece[] {
 }
 
 export async function parseMarkdown(source: string, root: string, sourcePath: string): Promise<ParseResult> {
-  const parts = matter(source);
+  const parts = matter(source, {});
+
   // TODO: We need to know what line in the source the markdown starts on and pass that
   // as startLine in the parse context below.
   const md = MarkdownIt({html: true});