add parameter for prefix on master editors' file
modify templates that handle author, editor, and principal in bibliographic contexts to test for the presence of a ref attribute containing the standard prefix for the master editors' file
add parameter for the relative pathname to the master editors' file
add variable that stores the master editors' document tree at the beginning of the transform process
modify templates that handle author, editor, and principal in bibliographic contexts to also handle the person element; modify the conditionals inside them that detect cross-references to the master editors' file to actually do the lookup in that file and process the target element, rather than the referring element
new run of all content with modifications to pull editors' info from the master editors' file; fixes #53