Allow LDR to act as a control field in MarcExtractor

Adds code and tests to allow the string 'LDR' to act as a control field but pulling data from the marc record leader in a MarcExtractor specification.

Accomplishes this by checking for the 'LDR' string as the "tag" in the spec, and creating a special control-field-quack-alike to represent the leader data when necessary.

Will totally hose anyone who actually has a 'LDR' control field (or variable field, for that matter); do we care?