Allow os-specific subsections

[JPHutchins]

This is breaking change. And easy to screw up.

Notably because it's possible to run any of the bash, zsh, and PS on any of Mac, Windows, Linux. So the OS needs to be checked from both the sh and powershell sections rather than simply assuming powershell == Windows, bash == Linux and zsh == Mac.

[path]
TOOL1=$HOME/tool1
TOOL2=$HOME/tool2

[path.windows]
# TOOL1 will not be overwritten on windows systems but TOOL2 will be.
TOOL2=C:/tool2

[JPHutchins]

Looks straightforward in PowerShell (not sure about PowerShell 5 🙄), simple booleans are ready:

$IsWindows
True
$IsMacOS
False
$IsLinux
False

In bash, $OSTYPE can be used to similar effect:

echo $OSTYPE
linux-gnu