Add a logging framework

[timholy]

This may help catch weird/rare errors. CC @pfitzseb, @vtjnash.

[codecov]

Codecov Report

Merging #163 into master will decrease coverage by 1.21%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #163      +/-   ##
==========================================
- Coverage   88.99%   87.77%   -1.22%     
==========================================
  Files           8        8              
  Lines         545      548       +3     
==========================================
- Hits          485      481       -4     
- Misses         60       67       +7

Impacted Files	Coverage Δ
src/Revise.jl	79.53% <100%> (-1.78%)	⬇️
src/types.jl	100% <100%> (ø)	⬆️
src/parsing.jl	94.91% <0%> (-3.39%)	⬇️
src/exprutils.jl	86.3% <0%> (-1.37%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 99d250a...314ec9e. Read the comment docs.

[vtjnash]

+1

Would it also be useful to sprinkle some @debug statements around? That allows someone to turn this on via the ENV (JULIA_DEBUG=Revise)

[timholy]

OK, I completely restructured this around Base's logging framework---not sure why I rolled my own when that was available.

However, I am not sure how to usefully turn on logging using the JULIA_DEBUG command. As you'll see if you inspect this, I exploit the Test.TestLogger and that's not the default. In the docs I show a more cumbersome way to turn it on, but if you have any other suggestions I'm all ears. (I am new to the logging system.)

[vtjnash]

Yeah, that’s a known problem (JuliaLang/julia#25404). You can also control that variable at runtime (via ENV)

[timholy]

Passed on Appveyor previously, but my AV queue is long (all the Images packages are running).