Starting a .yaml doc or an .md doc with --- erroneously triggers a new line indentation

[mikestopcontinues]

Description

The line after --- should not be indented. Yet this bug causes an indent when you trigger a new line.

Steps to Reproduce

1. Enable language-yaml "Auto Indent"
2. Set the first line of document to --- (could be .yaml or .md)
3. Press enter.

Expected behavior: No indent on the new line.

Actual behavior: Indent on the new line.

Reproduces how often: 100%

Versions

OSX: 10.13.1
Atom : 1.22.1
Electron: 1.6.15
Chrome : 56.0.2924.87
Node : 7.4.0
apm 1.18.8
npm 3.10.10
node 6.9.5 x64
python 2.7.10
git 2.10.0

Additional Information

I have previously filed a bug with language-markdown as the bug exists in markdown files when EITHER plugin has "Auto Indent" enabled.

burodepeper/language-markdown#203