Deprecate DemuxLogger for TeeLogger, that does not support include_current_global kwarg

[oxinabox]

When looking at #20 I realized that including the current global logger often is not desired.
Since the default ConsoleLogger that comes with the REPL is not a pure sink,
and so one might prefer to instead point at a new ConsoleLogger(stderr, BelowMinLevel())

Plus the kwarg is longer than the function call,
plus in a deep compositional structure only at most one level would want that, so bad to default to true.

Deprecating that behavour out is hard, but deprecating to use a different logger with a new name is easy, and we wanted to change the name anyway.

Closes #1 by renaming DemuxLogger
Closes #11 by removing kwargs that interact with the global_logger

@c42f do you approve of the name? Was your suggestion originally,
or should we go for MultiLogger?

[codecov-io]

Codecov Report

Merging #22 into master will decrease coverage by 1.38%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
- Coverage   80.32%   78.94%   -1.39%     
==========================================
  Files           7        7              
  Lines          61       57       -4     
==========================================
- Hits           49       45       -4     
  Misses         12       12

Impacted Files	Coverage Δ
src/LoggingExtras.jl	100% <ø> (ø)	⬆️
src/tee.jl	87.5% <87.5%> (ø)

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[c42f]

I'm fine with the name TeeLogger, but neither do I really love it in the sense that I can't yet see it's an "obviously right" consistent and clear choice :-)

[oxinabox]

I am going to merge this and leave the issue open.