Customizable droplets processing

[Toparvion]

Is your feature request related to a problem? Please describe.
Sometimes it is desirable to alter jMint behavior in relation to droplets processing. For example, to indicate whether a droplet's applying is optional (or required) or to switch default logging stream from stderr to stdout. However, currently jMint has no means for setting anything up.

Describe the solution you'd like
Support a way to pass settings/properties to jMint alongside with droplets, e.g. by giving a *.properties file as one of the agent arguments.

Describe alternatives you've considered
It could also be done by means of JVM options (-D) but this seems less preferable as it doesn't allow to collect all the settings in a single file and makes the settings format a bit more tricky.

Additional context
Here will be a list of jMint features requiring customization via settings or properties:

• choosing of output stream for jMint logging;
• the isOptional flag for droplets (see Detect droplet target miss #9)