feat: 支持对 css 中的图片做 base64 处理

[Darmody]

resovle #448

[codesandbox-ci]

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Sandbox	Source
suspicious-agnesi-yrj7r	Configuration

[codecov]

Codecov Report

Merging #460 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #460      +/-   ##
==========================================
+ Coverage   97.31%   97.37%   +0.05%     
==========================================
  Files         295      295              
  Lines        3204     3274      +70     
  Branches      336      363      +27     
==========================================
+ Hits         3118     3188      +70     
  Misses         84       84              
  Partials        2        2

Impacted Files	Coverage Δ
packages/remax-cli/src/getConfig.ts	100% <ø> (ø)	⬆️
packages/remax-cli/src/defaultOptions.ts	100% <ø> (ø)	⬆️
packages/remax-cli/src/build/rollupConfig.ts	96.15% <ø> (ø)	⬆️
packages/remax-cli/src/build/plugins/postcssUrl.ts	100% <100%> (ø)	⬆️
packages/remax-cli/src/build/plugins/stub.ts	84.21% <0%> (-2.89%)	⬇️
...max-cli/src/build/plugins/fixRegeneratorRuntime.ts	100% <0%> (ø)	⬆️
...ax-cli/src/build/plugins/nativeComponents/index.ts	100% <0%> (ø)	⬆️
packages/remax-cli/src/build/plugins/template.ts	91.93% <0%> (+0.26%)	⬆️
packages/remax-cli/src/build/plugins/rename.ts	92.59% <0%> (+0.7%)	⬆️
packages/remax-cli/src/build/plugins/removeSrc.ts	92.04% <0%> (+1.01%)	⬆️
... and 1 more

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