Add a reader that accepts a file containing a file pattern

This lets us take advantage of FileStitcher as needed without explicitly
including it in the stack of wrapped readers.  Files ending in .pattern
are accepted, and should only contain a single line with the absolute or
relative file pattern (in the same format accepted by FileStitcher).

The primary motivation here is to help with importing the 2TB Dresden dataset.
Running ```echo 'spim_TL<1-600>_Angle<1-6>.tif' > spim.pattern``` in the same directory as the TIFF files and then importing ```spim.pattern``` should import everything as a single dataset (as in ImageJ).  The only caveat is that the file name detection currently sets the channel count to 6 - if it's more accurate to have 6 Images/series instead, then that will need to be fixed separately.

/cc @emilroz