RFE: Support external/custom version sources (plugins)

After inspecting nvchecker source code I found no way to include custom version handlers without core code modification as they seem to be hardcoded into handlers_precedence list and loading is always done from nvchecker.source package.

Instead of forking/rewriting/reinventing the wheel, I’d prefer to extend nvchecker with a simple plugin system:

• put custom version sources into a predefined user directory (~/.nvchecker/sources ?) OR custom directory specified with new CLI argument (--extra-sources my_nv_sources_dir)
• load all valid extra source handlers and use them the very same way as those included in nvchecker.source

That way, users can:

• use nvchecker and its nice version handlers alongside custom handlers to get versions from all possible sources including those that aren’t generic enough to have an upstream handler
• use/test custom handlers without forking nvchecker and optionally submit them easily for upstream inclusion

I’m willing and able to implement this if upstream is interested - please do let me know and I’ll start hacking. Otherwise I’ll be forced to create a competing project 😃

[lilydjwg]

A rewrite is ongoing to address several issues e.g. #132. Alongside those changes, I’m planning to use Python’s native namespace packages for plugins.

Please wait for a few days for a test version of 2.0 to come out. This will be a breaking-changes version and target Python 3.7+.

[lilydjwg]

The documentation is done!