Rewrite PKGBUILD.

Differences:

* Use `sha256sums` instead of `md5sums`.
* All variables use the curly brace syntax: ${...}. This syntax more clearly
  separates the variable from surrounding text.
* Rewrite the comment explaining why "src/github.com/dobyrch/" is created.
* Add a variable: _pkgname.
* Add a vim-specific comment at the bottom of the file, used for formatting.
* Change some double quotes to single quotes, remove other quotes entirely.
Rewrite readme. Change file mode.

Rewrite readme file to abide by an 80-character width limit. Make file
non-executable. That is, change mode from 0755 to 0644.
Update .gitignore.

Make git ignore vim swapfiles.