Add support for banner and footer

[Gelio]

This PR is an attempt to fix #482.

It adds support for banner and footer options, that allow for prepending/appending a user-provided string to each output file.
Similar to rollup's options.

I do not have full confidence that I have done everything correctly, as this is my first ever PR in go, which I began learning yesterday 😅

Things I did (hopefully correctly):

• add Banner and Footer options to configs
• add emitting those in the linker
• take account of the banner in the sourcemap
• add test cases for sourcemaps when banner and footer are specified
• add end-to-end test cases when banner and footer are specified
• add handling those options in the JS service

I'm looking for any feedback you have 🙂

Example

test.js:

console.log('hello world');

14:11 $ ./esbuild test.js --sourcemap --banner="/* Hello world */" --footer="/* footer */"
/* Hello world */
console.log("hello world");
/* footer */
//# sourceMappingURL=data:application/json;base64,ewogICJ2ZXJzaW9uIjogMywKICAic291cmNlcyI6IFsidGVzdC5qcyJdLAogICJzb3VyY2VzQ29udGVudCI6IFsiY29uc29sZS5sb2coJ2hlbGxvIHdvcmxkJyk7XG4iXSwKICAibWFwcGluZ3MiOiAiO0FBQUEsUUFBUSxJQUFJOyIsCiAgIm5hbWVzIjogW10KfQo=

Sourcemap visualization

[evanw]

Thanks for doing this. I haven't tested this extensively but this looks great as far as I can tell. Thanks for adding tests too. I can't think of anything else that would need to be integrated at the moment. I'll take care of documentation on the website.

[evanw]

Closing this PR. GitHub's merge UI just broke over the weekend so I had to merge manually, and it didn't end up closing this PR.

[Gelio]

Thanks for merging and taking care of the cleanup 👍 I'm glad and excited I could contribute