Add a default variant in addition to the original 256 one

[guillaumeboudon]

Why

I'm on MacOS, using base16 since a long time using the iTerm2, Vim and fzf templates.

I'm now using other terminal emulators and more and more Linux. Then I wanted to switch to this base16-shell template, in order to have all of my config depending on my shell config, instead of having having to handle some terminal emulators templates on one side, and the rest inside config files.

But, this base16-shell template only provids the 256 version, which is different from other templates. Then, I propose to implement the default behavior, in addition to the original 256 version.

How

I propose to add a $BASE16_SHELL_DEFAULT_VARIANT variable in order to switch to the 16 colors version. It is built not to break the current users configuration: unless this variable is set, the template acts exactly as it already does:

What

• Upgrade templates/default.mustache in order to change different colors depending on the $BASE16_SHELL_DEFAULT_VARIANT variable state
• Upgrade README.md to reflect new behavior
• Build the whole set of scripts/base16-*.sh, using the very last version of Base 16 Builder Python
• Adapt the colortest script to the new template

[JamyGolden]

@guillaumeboudon would you be willing to create a PR for this on https://github.com/base16-project/base16-shell? If you're not interested anymore I'll probably create a PR by cherry-picking your commits.