Add wrapper option

[drwpow]

Reason for change

It’s getting hard to support all the ways to expose these types. This adds the amazing --wrapper option that lets you do things like --wrapper "export namespace MyNamespace" or --wrapper "declare module '@api'". More details in the README.

Deprecates --namespace and --export flags in favor of something more versatile.

[codecov]

Codecov Report

Merging #14 into master will increase coverage by 1.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
+ Coverage   94.44%   95.55%   +1.11%     
==========================================
  Files           2        2              
  Lines          72       90      +18     
  Branches       18       33      +15     
==========================================
+ Hits           68       86      +18     
  Misses          4        4

Impacted Files	Coverage Δ
src/swagger-2.ts	95.23% <100%> (+1.2%)	⬆️
src/index.ts	100% <0%> (ø)	⬆️

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