feat: add plugin api for umi block

[yutingzhao1991]

api._modifyBlockDependenciesPackageJSONPath() // => /xxx/package.json
api._modifyBlockDependenciesCheckResult() // => { lacks[['antd', '^3.0.0']], conflicts: [['moment', '^2.3.0', '2.1.0']]}
api._modifyBlockContent() // => code

modify new api name: _modifyBlockPackageJSONPath, _modifyBlockDependencies, _modifyBlockFile

and support singular

[yutingzhao1991]

_modifyBlockDependenciesPackageJSONPath 用于修改安装物料 block 依赖是读取的 package.json 目录。这个名字会不会太长...

[coveralls]

Pull Request Test Coverage Report for Build 1949

• 11 of 32 (34.38%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.4%) to 33.419%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/umi-build-dev/src/plugins/commands/block/block.js	10	31	32.26%

Files with Coverage Reduction	New Missed Lines	%
packages/umi-test/src/index.js	2	0.0%

Totals
Change from base Build 1930:	0.4%
Covered Lines:	1152
Relevant Lines:	3512

---

💛 - Coveralls

[sorrycc]

建议 _modifyBlockPackageJSONPath, _modifyBlockDependencies, _modifyBlockFile

[sorrycc]

only 去掉。

[yutingzhao1991]

去掉了，之前不小心提到 master 了，这个是去掉的。