[Bug] KeyError: 'Slug'

[Kristinita]

1. Summary

If users set [**SLUGIFY_SOURCE = 'basename'**](http://docs.getpelican.com/en/stable/settings.html?highlight=slugify_source#basic-settings) in pelicanconf.py, Slug is not mandatory parameter.

2. Expected behavior

Not stack trace in console.

3. Actual behavior

command: pelican_insert_metadata
Traceback (most recent call last):
  File "D:\Sublime Text 3 x64\sublime_plugin.py", line 818, in run_
    return self.run(edit)
  File "Pelican in D:\Sublime Text 3 x64\Data\Installed Packages\Pelican.sublime-package", line 187, in run
KeyError: 'Slug'

4. Settings

Example of User/Pelican.sublime-settings file, I remove Slug parameter:

{
    "article_metadata_template": {
    // Metadata template for Markdown articles
    "md":
      [
        "Title: ",
        "Template: ",
        "Date: %(date)s",
        "Modified: %(modified)s",
        "Author: %(author)s",
        "Lang: %(lang)s",
        "JQuery: true",
        "DetailsPolyfill: false"
      ]
      }
}

5. Steps to reproduce

The problem is reproduced for me in a version of Sublime Text without plugins and user settings.

I open any md file → Ctrl+Shift+P → Pelican: Insert Metadata → I get actual behavior.

6. Environment

Operating system and version: Windows 10 Enterprise LTSB 64-bit EN Sublime Text: Build 3126

Thanks.