Polish wrapping into the standard logger

[zonescape]

Continuing #7. I think that current implementation of wrapping into the standard logger can be simplified a bit:

• reduce exported funcs/types to just ToStdLogger()
• make use of standard logger's prefix

[umputun]

Having io.Writer adopter can be useful by itself and I've exposed it after some thinking about possible use cases.

[zonescape]

I've exposed it after some thinking about possible use cases

Note that current implementation of lgr.Writer relies on this property of log.Logger: "Each logging operation makes a single call to the Writer's Write method". Other clients may not hold this property.