Give canvas tag an explicit size.
Give canvas tag an explicit size.

Fixes #8

- Set the size in px of the canvas during resize. This avoids having the
  canvas stretch when the size of the window does not exactly line up
  with the expected size.
- When handling a resize, only change the size of the window if the
  window size doesn't match what is expected for the current number
  of rows/columns
- Fill the background of the window with the theme background color
dna113p/neovim-e,demelev/neovim-e,coolwanglu/neovim-e