Create a new `collector` folder to store the code for the different
collectors. Move the existing f2b and textfile collectors to this folder.
Minor refactors to the f2b collector to better match the code style of the
newer textfile collector.
Add support for collecting arbitrary metrics from a textfile as well as
metrics collected from fail2ban. This allows other data to be exported
along with the fail2ban metrics (e.g. instance metadata).
Update the docker image to allow mounting a folder with a collection of
metric files to be exported. Only files ending in `.prom` with be read.
Update project README with the new functionality.