Support command line wildcards for GPX files

[McPringle]

It would make loading a bunch of GPX files easier, if you could write something like:

java -jar gpx-animator.jar --input-files *.gpx

The shell will automatically expand the wildcard, so the real command line would look like:

java -jar gpx-animator.jar --input-files file1.gpx file2.gpx file3.gpx

GPX Animator could easily be extended to support multiple file names.

Idea from @willjobs based on #314.

[McPringle]

Maybe this is easier to implement when by definition input files with wildcards need to be specified as the last parameter.

[ky0n]

i'm working on this