User configurable markdown file extension list

[gittycat]

Goal

To allow users to override the default file extensions that are recognised as markdown by Glow.
Requested in #732

Issue

The list of extensions is currently hardcoded in two places (utils.go and ui.go) and there is no way for a user to override or add to this list.

Proposed Solution

Two changes are proposed:

1. Add a new markdownExtensions list field to the config file that overrides the defaults.
   These two lines would be added to config.yml when it is first created.

# recognized markdown file extensions
markdownExtensions: [".md", ".mdown", ".mkdn", ".mkd", ".markdown"]

For existing users, config.yml is not modified. They can manually add it later to override the defaults.

1. Add a flag, -f / --force to force markdown processing. This would be used for quick or one off display of markdown files with unconventional extension without having to change the config.

Eg:

glow -f input.txt

Note

The above two changes should not break compatibility.

My fork includes a commit with the changes needed
I'm happy to create a PR if maintainers like the idea.