Add support for older neovim version.

Switch to using custom base64 encode function to avoid running into problems with vim.base64 on older neovim versions where it doesn't exist.

Add more options

Add new options for setting up PicVim

• It is backwards compatible and should not break any changes
• Adds the option to set filetypes to display as image.

Initial Release 🎉

Hello Vimmers!

This is the Initial Release of PicVim with all the basic functionality.

Enjoy Images right in your favorite editor - Neovim! 🎉