Allow custom drill decimal format

In altium you can specify a whole range of different formats (eg 4:4), and the default does not seem to be the same as excellon's default.
Here I allow the format to be passed in via the options.  Not sure if you'd like to handle this in another way?
Another/additional option: Altium includes a comment that contains the format in the file, here's a branch that extracts it from the comments if it's present - https://github.com/circuithub/gerber-to-svg/tree/feat/parse-drill-format