Ignore initial whitespace in a continuation line for a metadata field

When parsing a metadata field that spans several lines skip over the
initial whitespace on each line.  This allows alignment of metadata
fields:

    ---
    description: A long description that would look better if it
                 spanned multiple lines and was indented
    ---
Merge pull request #115 from pjones/master

Ignore initial whitespace in a continuation line for a metadata field