Configurable string formatter

[solnic]

This adds support for template option when setting up loggers:

irb(main):001:0> logger = Dry.Logger(:hello, template: "OMG IT'S A LOG: %<message>s")
=> 
#<Dry::Logger::Dispatcher:0x00000001049b4010
...
irb(main):002:0> logger.info("hello")
OMG IT'S A LOG: hello
=> true

Following tokens are supported:

• message
• severity
• time
• progname