Add support for imports with line-breaks

[trotzig]

As reported by @lencioni in #9:

If I have a section of required modules at the top that looks like this:

const MyModule = require('path/to/my_module');
const MyModuleThatHasALongName =
  require('path/to/my_module_that_has_a_long_name');
const SomeModule = require('path/to/some_module');

import-js gets confused when adding a new module to the list:

const AnotherModule = require('path/to/another_module');
const MyModule = require('path/to/my_module');

const MyModuleThatHasALongName =
  require('path/to/my_module_that_has_a_long_name');
const SomeModule = require('path/to/some_module');

where I would expect:

const AnotherModule = require('path/to/another_module');
const MyModule = require('path/to/my_module');
const MyModuleThatHasALongName =
  require('path/to/my_module_that_has_a_long_name');
const SomeModule = require('path/to/some_module');

It would be nice if import-js worked better with this format.

I've added basic support for line breaks in this commit. I say basic
because I expect there to be edge cases that I haven't thought of.
We use simple regexs to parse the js file content, and like all simple
regex parsers, they are bound to fail. However, I don't want to bring in
a js parsing framework just yet. So in this commit I extend the regex
approach with some logic to deal with possible line-breaks.

One idea also listed in #9 was to add support for import-js to
automatically line-break long lines. I haven't made that change in this
commit (new imports are still one-line), but I might do that later if I
find time.

[trotzig]

I'm going to merge this as is. @lencioni those are all valid comments but I don't have time to address right now. Thanks for taking the time to review!

[lencioni]

Sounds good. I'll send you a PR with these changes then.