Fewer dirs open

[luhring]

Hello! This PR does the following:

1. It introduces a minimal amount of benchmarking. I came across issue any benchmark #19 after noticing some slowness in the implementation (for how we're using it specifically). There's certainly more that can be done with benckmarks, but I see from reading the issue that performance isn't a key criterion for doublestar — so feel free to discard this part!
2. It introduces an executable entrypoint for the Glob function, just so that it's easy to examine behavior of the function quickly from the shell, without needing to incorporate the function into a larger project. For example, you can run go run ./examples/find.go '/Users/dan/**/*.go'.
3. It addresses an issue where, as directories were being traversed, directories remain open even after the contained files had been read. Every opened directory was ultimately closed, but because the call to dir.Close() was deferred, and the function body was large enough that recursive operations were being performed within the same function, directories ended up remaining open longer than necessary.

[codecov]

Codecov Report

Merging #47 into master will decrease coverage by 0.93%.
The diff coverage is 63.63%.

@@            Coverage Diff             @@
##           master      #47      +/-   ##
==========================================
- Coverage   80.55%   79.62%   -0.94%     
==========================================
  Files           1        1              
  Lines         432      319     -113     
==========================================
- Hits          348      254      -94     
+ Misses         60       40      -20     
- Partials       24       25       +1     

Impacted Files	Coverage Δ
doublestar.go	79.62% <63.63%> (-0.94%)	⬇️

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[bmatcuk]

Hi @luhring! Thanks for the PR! Makes sense to me... I appreciate that you broke all the changes into individual commits, too! I merged in your changes to close directories, and your example program. I left out the changes to gitignore and your benchmarks - but only because I have a branch I've been working on that has benchmarks. I also back-ported your code to the v1 branch =)