Restore ability to compile to JavaScript.

[dmitshur]

Current web-specific code implementation is not complete, and assumes non-east-asian. This is still an improvement because at least it compiles. But someone who has access to and is familiar with east-asian systems can complete the functionality.

This fixes the issue where I can no longer compile markdownfmt package to JavaScript, because it imports go-runewidth.

The posix solution uses os.Getenv, which doesn't work because "os" package imports "runtime", which is not fully supported.

[coveralls]

Coverage remained the same at 90.15% when pulling ffb40a661c1496498add9254d4507d55dc92b7ac on shurcooL:master into 8adae32de8a26f36cc7acaa53051407d514bb5f0 on mattn:master.

[mattn]

Thank you

[dmitshur]

Thanks!