Implement Npm.strip, for use in package.js.

Summary:
The `Npm.strip` method makes up for packages that have missing or
incomplete .npmignore files by telling the bundler to strip out certain
unnecessary files and/or directories during `meteor build`.

The `discards` parameter should be an object whose keys are NPM
package names and whose values are arrays of strings (or regular
expressions) that match paths in that package's directory that should be
stripped before installation. For example:

  Npm.strip({
    connect: [/*\.wmv$/],
    useragent: ["tests/"]
  });

This means (1) "remove any files with the `.wmv` extension from the
'connect' package directory" and (2) "remove the 'tests' directory from
the 'useragent' package directory."

The values can also be objects, in which case the keys of the nested
objects will match nested dependency names. For example,

  Npm.strip({
    connect: {
      multiparty: ["test/"]
    }
  });

will discard the "test" directory from the "multiparty" package that is
depended upon by the "connect" package.

If you need to discard files from both "connect" and "multiparty", here's
a little trick you can use:

  Npm.strip({
    connect: {
      connect: ["huge.wmv"],
      multiparty: ["test/"]
    }
  });

This makes intuitive sense because requiring the "connect" package from
the "connect" package always returns the package itself.

Test Plan:
Run `meteor rebuild <package>` for the packages whose package.js files I
modified, and verify that the `Npm.strip`ped paths are absent from the
generated bundle.

Reviewers: glasser, nim, emily

Differential Revision: https://phabricator.meteor.com/D865
Use Npm.strip to remove some extraneous large files from core packages.

So far I have only removed files from packages that are included by
default in newly `meteor create`d projects. We should probably make a pass
over all the core packages, too.