Deprecate `shell.SHELL.VAR` usage in favor of `VAR.SHELL`

[superatomic]

Currently, to declare shell specific variables (#4, implemented in #7), this syntax is used:

[shell.bash]
HISTFILE = "$HOME/.bash_history"

or this:

shell.bash.HISTFILE = "$HOME/.bash_history"

This has a couple of problems:

• Writing this syntax out on one line like shell.SHELL.VAR is verbose.
• While it's a minor problem, it is impossible to set a variable named shell.
• All shell specific variables are set last, not in the order in which they appear in the file.

To fix this problem, this new syntax will be implemented that looks like this:

HISTFILE.bash = "~/.bash_history"

Up until the v1.0.0 release, the old syntax will still be usable, but will emit warnings.