feat: support customized package key

[VineyAT]

• support registering customized package key which is the prefix of cache key. You can empty it as well.
• refactor naming in options

[codecov]

Codecov Report

Merging #14 (f6bdc3b) into master (375a308) will increase coverage by 0.21%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
+ Coverage   90.01%   90.23%   +0.21%     
==========================================
  Files          11       11              
  Lines         541      553      +12     
==========================================
+ Hits          487      499      +12     
  Misses         30       30              
  Partials       24       24              

Impacted Files	Coverage Δ
event.go	82.69% <100.00%> (ø)
factory.go	100.00% <100.00%> (ø)
interface.go	100.00% <100.00%> (ø)
key.go	100.00% <100.00%> (ø)
options.go	100.00% <100.00%> (ø)

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