Add multiple index support

[csw]

In many cases such as the UCSC data sets, MAF data can be broken up into many individual files. It would likely be useful to present a unified interface to such MAF data, rather than making the programmer find and open the correct MAF file and corresponding index.

There is also a question of whether it would be better to use one index file per MAF file, or create one big index per directory / data set / whatever. The latter does raise the possibility of concurrent reads and writes, which is not the end of the world but should be taken into account.

[csw]

Using one index per MAF file so far.