[improvement] Flag for `.fpm` file and allow comments in `.fpm`

[aanno]

Dear @jordansissel ,

great tool! I've got 2 ideas for improvement:

1. A flag for giving a fpm-config (e.g. --fpm-config) would be great in addition to rely on the convention that the file is called .fpm.
2. Please allow for comments in the .fpm file (e.g. lines beginning with '#').

Kind regards, aanno