Add number prefix to each buffer name

[ruiheng]

In my branch, I added number prefixes to each buffer name, begin with 1. This number is controlled by format string '%o'.

The point is that, for example, with proper key mapping, I can switch to the first buffer with key <leader>1, key <leader>0 to the 10th buffer, and I know exactly which number key to use by looking at the number prefix.

Some my considerations:

• Numbers are shown in smaller size, which may make it easier to recognize.
• Order number begins with 1 instead of 0, because I think key 1 is easier to press than 0.
• Numbers over 10 have not been shown because I cannot find proper key mappings for switching to those buffers. So I think it is unnecessary to take extra screen space.
• I did not modify doc because my English is not good enough.

Hopefully this small enhancement can be useful to others.

[zefei]

Hey @ruiheng, thank you for the PR. I like the idea and your considerations, but I can't merge it for a few reasons:

• the function signature on public functions are not supposed to change as they become part of the API. wintabs#renderers#buf_label is used in vim-wintabs-powerline, so it can't be changed in backward-incompatible way.
• the small number looks really nice. However, having this option and %n seems strange as one uses superscript numbers and one uses regular. I think it's better to have them render the same style, probably configurable between superscript and regular.

I'd love to merge your PR if you can find ways to walkaround these issues. Thank you for taking time on this!

[ruiheng]

I didn't know vim-wintabs-powerline before.

Since I am not familiar with VimL, I may not find any workarounds in short time.

Thanks.

[zefei]

Closing this since it's unlikely to be mergeable in its current state. Thank you for doing this! I'll try to include index number the next time I change renderer API.