feat(weex): add code coverage test

[imyzf]

1. Add rollup-plugin-istanbul to instrument weex package build, this let us see the coverage of each source file rather than the built file.
2. Add nyc package to call istanbul from cli.
3. Add .weex.nycrc to config nyc for weex.
4. Add npm script test:weex:cover to run the test.

Here is the result from coverage/lcov-report/index.html:

Emmmm... the coverage is very low now, and I'll write more test case in the next time .

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What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
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[Hanks10100]

I have tried this test script, it works well on my machine. Good job! However, I think it's better to put the .weex.nycrc file within the test/weex folder rather than the root path.

[imyzf]

I have added some test case and ignore some line. Now it is: