add support for Vue.extend vue-i18n instance

[jaredzhu1993]

To fix #200 , now we can use

const i18n = new VueI18n({ ... })
const Constructor = Vue.extend({ i18n })
const vm = new Constructor({ ... }).$mount()

[codecov-io]

Codecov Report

Merging #420 into dev will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             dev     #420      +/-   ##
=========================================
+ Coverage   95.9%   95.91%   +<.01%     
=========================================
  Files          9        9              
  Lines        635      636       +1     
=========================================
+ Hits         609      610       +1     
  Misses        26       26

Impacted Files	Coverage Δ
src/install.js	100% <100%> (ø)	⬆️

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[kazupon]

Good job! 👍
Thanks!