Added a more API-like interface to pbjs and pbts

[cromefire]

For easier usage in things like gulp

[cromefire]

You can now do:

import {pbjs} from "protobufjs/cli/pbjs";
import {promisify} from "util";
const pbjsAsync = pomisify(pbjs)

pbjs({encode: false}, {content: "syntax = \"proto3\";", name: "foo.proto"}, () => {
    // do something
})
// or
async function doSomething() {
    const js = await pbjsAsync({encode: false}, {content: "syntax = \"proto3\";", name: "foo.proto"})

    // do something
}

Same for pbts

[cromefire]

Also the main functions are not affected by this, so there are no breaking changes

[cromefire]

Is there anything blocking this from being merged @alexander-fenster ?

[alexander-fenster]

@cromefire Just the lack of my free time - apologize for that! I'll go ahead and review/merge later this week. I do use pbjs from my code using the existing API (just the command line options as an array) so I appreciate these changes, just need some time to read through the code :)

[cromefire]

The best thing is that you don't have to save the file, but you can retrieve it as Buffer