Renamed a significant part of the API to be more julian

[rofinn]

Closes #31

Main changes include:

1. Removing _ from public method names
2. Adding ! to mutating methods
3. Removing add_handler and remove_handler in favour of push!(logger, handler). I'll be replacing the internal handler Dict with a Vector in a future release.
4. Overloaded += for adding handlers and filters to loggers (or filters to handlers).

[codecov[bot]]

Codecov Report

Merging #51 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #51   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           8      8           
  Lines         200    211   +11     
=====================================
+ Hits          200    211   +11

Impacted Files	Coverage Δ
src/deprecated.jl	100% <ø> (ø)	:arrow_up:
src/Memento.jl	100% <ø> (ø)	:arrow_up:
src/handlers.jl	100% <100%> (ø)	:arrow_up:
src/loggers.jl	100% <100%> (ø)	:arrow_up:

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[rofinn]

The argument around + is that it fits with some other packages in the julia ecosystem (e.g., adding constraints in Convex.jl).

[iamed2]

Convex uses that out of deference to MATLAB syntax used in its predecessors. Julia has many more options available including functions that mutate their arguments (+ was used in MATLAB because all MATLAB functions took arguments as copy-on-write).

[rofinn]

Ahh, okay, I didn't realize that choice was based on MATLAB syntax. I did notice that having a shorthand for push!(logger, handler) was nice when you have complicated handler constructors, but I suppose that isn't necessary for this PR. Maybe @omus has an opinion on this?

I'll note that several other languages have a shorthand for pushing elements onto a list:

swift: names += ["Fred"] haskel: names ++ ["Fred"] ruby: names << "Fred" ocaml: names@["Fred"] julia: names <! "Fred"?

[omus]

I think we can just stick with push! since a mutating += is a little weird (I was alright with it originally since Convex.jl was doing it). I think having a shorthand would be nice but I we can open an issue and wait until we come up with something solid.

[rofinn]

@iamed2 and @omus Is this good to merge?