Ability to pass several source files with `-s` option

[ddosia]

My use case is to combine several json files into one hrl.

I am doing this by several jerg invocations and redirecting output to one file. The problem is output contain ==> jerg (generation complete) strings and a lot of header comments (which I am cutting with awk).

I suggest to change -s option to take several files or by passing -s several times. If you don't mind, I could send PR.