Always display code view in GitHub

[synthead]

Fixes https://github.com/tonchis/vim-to-github/issues/16!

This PR adds ?plain=1 to the URLs so that GitHub always opens in the code view. This is helpful when working in markdown, for example, because GitHub would simply display the HTML-rendered markdown otherwise.

From https://github.com/tonchis/vim-to-github/issues/16:

When viewing a markdown file, if I run :ToGithub, I get a URL like:

https://github.com/tonchis/vim-to-github/blob/9ea9c75b6cd48bd42823a39c56a05a2ff8161536/README.md#L3-L3

However, this link brings up the HTML representation of the markdown file, not the lines of code:

To always make the permalink open the lines of code, we want to add ?plain=1 to the URL, like so:

https://github.com/tonchis/vim-to-github/blob/9ea9c75b6cd48bd42823a39c56a05a2ff8161536/README.md?plain=1#L3-L3

This is the same as clicking the Code button without ?plain=1, and ?plain=1 also enables the "code" mode:

This also works well on files that don't have any "viewer" in github that would default to the "code" view. Plus, it's more explicit, which I think is a good thing. Here's an example:

https://github.com/tonchis/vim-to-github/blob/9ea9c75b6cd48bd42823a39c56a05a2ff8161536/plugin/to-github.vim?plain=1#L46-L49

And here's the link directly placed in the issue, where you can still see that the snippet is displayed:

https://github.com/tonchis/vim-to-github/blob/9ea9c75b6cd48bd42823a39c56a05a2ff8161536/plugin/to-github.vim?plain=1#L46-L49

We should add ?plain=1 for all generated links! This will make the links always go to lines of code, no matter what :ok_hand: