Skip untranslated posts in archive if config says so (#1360)
doh
merged master
Merge branch 'master' into fix-1360
oops^2
Initial commit to use ghp-import in github_deploy.

The tool automatically rebases before each time it pushes.  We will need
to remove a config variable that currently makes this optional.
Remove unused imports.
Remove the GITHUB_PULL_BEFORE_COMMIT option.

ghp-import always tries to rebase before commits.
Don't set unnecessary attributes on CommandGitHubDeploy.

Minor refactor.
Set default values for github deploy config variables.
Add a note on installation of ghp-import.

[skip-ci]
Clarify location of `custom.css` file

The current documentation doesn't make it clear that the path of `custom.css` file refers to a path within the `files` directory, so when I tried to create a custom CSS file it wasn't picked up.

Not sure if this change is the clearest way of expressing the correct location but it seemed perhaps better than "you can create a ``assets/css/custom.css`` file in the ``files`` directory and then..."
Further improve wording of custom.css file placement

As per: https://github.com/getnikola/nikola/pull/1512#issuecomment-64617067
Merge pull request #1512 from follower/patch-1

Clarify location of `custom.css` file
Add ghp-import to requirements.txt

[skip ci]
Add versioned requirement for ghp-import.
Update the docs for github_deploy.

[skip ci]
Merge pull request #1509 from getnikola/ghp-import-deploy

Use ghp-import in the github_deploy command.
Move ghp-import requirement to extras.

[skip ci]
Merge branch 'master' into fix-1360

Conflicts:
	CHANGES.txt
	nikola/plugins/task/archive.py