Replace ConstantsFlags with Config

[CraZySacX]

• Deprecated gen and ConstantsFlags (will remove in version 5).
• Added Config to control instruction generation. The bitflags were getting out of hand so switching to the Config pattern seemed prudent.
• Added support for timezone specification on the various date/time outputs (Utc and Local supported).
• Added support for various date/time/timestamp output combinations to suit different needs.

[codecov]

Codecov Report

Merging #51 (758060f) into master (f49246c) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master       #51    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           10        10            
  Lines          502       810   +308     
==========================================
+ Hits           502       810   +308     

Impacted Files	Coverage Δ
src/constants.rs	100.00% <ø> (ø)
src/feature/mod.rs	100.00% <ø> (ø)
src/lib.rs	100.00% <ø> (ø)
src/config.rs	100.00% <100.00%> (ø)
src/feature/build.rs	100.00% <100.00%> (ø)
src/feature/cargo.rs	100.00% <100.00%> (ø)
src/feature/git.rs	100.00% <100.00%> (ø)
src/feature/rustc.rs	100.00% <100.00%> (ø)
src/gen.rs	100.00% <100.00%> (ø)

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