Added ability to disable output even if feature is enabled

[CraZySacX]

• Added disable_* function to EmitBuilder to allow disabling output even if feature is enabled.

Fixes #201

[codecov]

Codecov Report

Merging #208 (a66897c) into master (e802dab) will increase coverage by 0.20%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #208      +/-   ##
==========================================
+ Coverage   99.27%   99.48%   +0.20%     
==========================================
  Files          23       23              
  Lines        1520     1541      +21     
==========================================
+ Hits         1509     1533      +24     
+ Misses         11        8       -3     

Impacted Files	Coverage Δ
vergen/tests/build_output.rs	100.00% <ø> (ø)
vergen/tests/cargo_output.rs	100.00% <ø> (ø)
vergen/tests/git_output.rs	100.00% <ø> (ø)
vergen/tests/rustc_output.rs	100.00% <ø> (ø)
vergen/tests/sysinfo_output.rs	100.00% <ø> (ø)
vergen/src/emitter.rs	94.89% <100.00%> (+0.92%)	⬆️

... and 1 file with indirect coverage changes

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