I18n JS parser: support precompiled assets

This patch makes `i18n:check` and `i18n:generate_js` able to parse
source files that may contain more than one AMD module. Usage is
described in the gem's (newly added) README.md file.

Closes CNVS-14691

TEST PLAN
---- ----

  - make sure `i18n:generate` and `i18n:check` rake tasks work

Change-Id: Ic1280c0b31fd274da2529ebeac7f72f72934f1ab
Reviewed-on: https://gerrit.instructure.com/39014
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Jensen <jon@instructure.com>
Product-Review: Jon Jensen <jon@instructure.com>
QA-Review: Jon Jensen <jon@instructure.com>