fix: Honor eager setting (false by default)

[coreyfarrell]

This delays loading of istanbul-lib-instrument until it is needed.

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This fixes a performance regression caused by #985, adds tests to prevent in the future. To test you can create a script eager.js:

console.log(Object.keys(require.cache).length - 1); // don't count eager.js itself

Then you can test the effect of non-eager loading:

$ node eager.js
0
$ ./bin/nyc.js --silent --exclude=eager.js --eager=true node eager.js
293
$ ./bin/nyc.js --silent --exclude=eager.js node eager.js
143

Running under nyc with eager: true (or without this patch) results in 293 additional modules being loaded, this patch by default reduces the "always load" to 143. This is demonstrated here by ensuring that all sources are excluded from instrument but in practice large test suites that load sources from multiple processes would see benefit when caching-transform bypasses repeated instruments.

[coveralls]

Coverage increased (+0.2%) to 98.715% when pulling 25730d5 on coreyfarrell:fix-eager into 03ba5b9 on istanbuljs:master.