[add] github actions

[get-me-power]

According to an email from travis-ci, travis-ci.org will be officially closed down completely on December 31st, 2020
Therefore, We should consider migrating vim-airline's CI.

I wrote a PR to add github-actions because I thought it was less restrictive and faster to build with github-actions than travis-ci.com.

[get-me-power]

Strangely enough, only vim8.2.0000 does not terminate the test successfully...

[get-me-power]

@chrisbra

note: I'm using action-setup-vim, which simplifies setting up the Vim environment in Github Actions, and there are two types of it. In vim-airline, use thinca/action-setup-vim.

• https://github.com/thinca/action-setup-vim

• https://github.com/rhysd/action-setup-vim

I think the difference is that the former uses cache and the latter does not.
I used the former, but faced the problem that vader.vim did not work well with vim8.2.0000.
As a test, I used the latter, action-setup-vim, which does not have a cache, and the test was confirmed to finish successfully.
My guess is that the cached vim is incompatible with vader.vim (not exact information).
However, since the latter has been tested successfully with the same process that was done in travis, why not apply the latter this time?

[chrisbra]

I mean, I know what they are (that is indeed one of the parts of code I hacked to remove the paths from the tab captions), but what would you suggest to have that custom formatter to do in order to solve this?

sure. I'll merge it, as it looks like you have finished transitioning this to github actions. If there need to be further actions, please open a new PR then.

Thanks, as always.