Support multi-line strings

[tuyakhov]

Thank you for creating this library and saving us a lot of time. I really appreciate your effort.
There is a problem that we experienced while implementing continuous deployment on one of our projects.
One of our .po files contained a multi-line string which led to creating an invalid translations.js file. Here is an example of the content in that file:

msgid "Apply"
msgstr "Siguiente\n"
"Aplicar"

This is a correct string and supported by [PO file format].(https://www.gnu.org/software/gettext/manual/html_node/PO-Files.html)
Since the newline character is not escaped when running 'i18n_import' script the produced result is an invalid Javascript code and it will look like this:

export const translations = {
  'es': {
     'Apply': 'Siguiente
Aplicar'
  }
}

[francescarpi]

Thanks! :)