Add CDX 'migration' tool to wb-manager

[ikreymer]

Allow users who have substantial existing cdx indexes to easily migrate them to the preferred format. Users shouldn't have to know what kind of format their index is in (as discussed in #79), and this tool would quickly convert existing cdx files to conform to the proper format (probably cdx-json).

Usage could be:

wb-manager migrate-index [<path/to/cdx>]

If no path provided, recursively look for all .cdx files in current working dir.

[ikreymer]

Command syntax is: wb-manager convert-cdx <path/to/cdx>