Allow webpack 2 and 3 as peerDependencies

[gabro]

Despite the changes in webpack 2, this plugin is still needed in some instances (see for example the discussion here: TypeStrong/ts-loader#108).

I've tested it with webpack 2 and 3 and it works as intended.

[TiddoLangerak]

I'll try to find some time to look into this soon(ish). I got to say thought that to me it sounds like a problem with the ts-loader. Webpack 2 & 3 have proper functionality to make the build fail with a non-zero exit code, which other loaders manage to use just fine. Do you happen to know if other loaders experience similar issues?

[gabro]

I don't know any other, but if you read the linked issue (you can go directly to the latest messages) it appears that webpack 2/3 still has some hiccups with errors and exit codes.

[pksunkara]

I support this. Webpack 2/3 is still having issues.

[TiddoLangerak]

I unfortunately don't have time to investigate this fully, but since this is apparently an issue for more people I'll merge this PR and make a new release.

[gabro]

Thank you @TiddoLangerak, appreciate it :)