use template to send mail for export job completion

[Jaspreet-singh-1032]

Closes #83

Allow users to override the template and subject that is used to send export job completion mail.

Default values:-

Default email subject: "Django: Export job completed"
Default email template: "email/export_job_completion.html"

To override default values override these variables from the Django settings:-

EXPORT_JOB_COMPLETION_MAIL_SUBJECT="Custom subject"
EXPORT_JOB_COMPLETION_MAIL_TEMPLATE="myfolder/custom_template.html"

[Jaspreet-singh-1032]

Hi @timthelion, Please have a look into this PR and let me know if any changes are needed.
Thanks!

[timthelion]

This looks excellent. Two small issues:

1. You should add the html2text dependency here as well:

   django-import-export-celery/setup.py

   Line 9 in afe519b

   requires = ["Django", "django-import-export", "django-author"]

2. It seems you have some flake errors

[Jaspreet-singh-1032]

Sure, will address them.