兼容webpack4.0

ERROR in ./node_modules/_alimask@1.0.3@alimask/dist/alimask.min.js
Module not found: Error: Can't resolve 'canvas' in '/Users/v/Work/tests/mcrm/node_modules/_alimask@1.0.3@alimask/dist'
@ ./node_modules/_alimask@1.0.3@alimask/dist/alimask.min.js 27:16-40
@ ./components/water-mask/index.js
...
ℹ ｢wdm｣: Failed to compile.

升级到webpack4+后, 会报上述错误, 查了一下, 在src/alimask.js里做如下改动即可解决这个问题:
line 56:
var Canvas = module.require('canvas'); => var Canvas = eval("module.require('canvas')");
如果有时间麻烦改下, thx

Start with src/alimask.js at line 56 and reproduce the reported webpack 4+ build failure involving alimask.min.js and the unresolved canvas module. Verify the compatibility change there, then confirm the water-mask component compiles successfully under webpack 4+.

Written by the indexing model from the issue text.