Allow 'update on save' to work on .txt files, and allow this to be customised in settings

[0xdevalias]

Purpose

Currently update on save happens for 'Markdown' files:

• https://github.com/naokazuterada/MarkdownTOC#automatic-refresh-of-toc-when-markdown-document-is-saved

Which is defined as:

• https://github.com/naokazuterada/MarkdownTOC#supported-file-extensions

  • Make sure your file's extension is in the following list. .md .markdown .mdown .mdwn .mkdn .mkd .mark

How important it is

Would be pretty useful.

Ideal implementation

Being able to configure what extensions/file types the 'update on save' works on in the package settings would make the most sense to me.

.txt doesn't need to be added as a default, but being able to manually configure it would be helpful.

Currently this list seems to be hardcoded in markdowntoc/autorunner.py:

https://github.com/naokazuterada/MarkdownTOC/blob/b61546d001661d9385423556a62c21c36abc6857/markdowntoc/autorunner.py#L5-L11