Use UTF-8 as default content type for YAML Files

[tfesenko]

Extracted from https://github.com/RepreZen/SwagEdit/issues/158

We may need to modify YEdit. @tfesenko to check if YEdit already provides a content type for YAML file (I think it does, just need to verify it).

Here's a page that describes how plugins can define new content types: http://help.eclipse.org/mars/index.jsp?topic=%2Forg.eclipse.platform.doc.isv%2Fguide%2Fruntime_content_contributing.htm

In https://github.com/oyse/yedit/blob/master/org.dadacoalition.yedit/plugin.xml

    <extension
        point="org.eclipse.core.contenttype.contentTypes">
        <content-type
            base-type="org.eclipse.core.runtime.text"
            file-extensions="yml,yaml"
            id="yamlsource"
            name="Yaml Content Type"
            priority="high">
        </content-type>
    </extension>