Command execution time indicator, performance enhancements, and more

• There is now a command execution time indicator.
• There are more psvar elements available to custom prompts.
• By popular demand, it is now possible to use AGKOZAK_LEFT_PROMPT_ONLY=1 with AGKOZAK_MULTILINE=0, although the two options together may be visually unappealing on a slow system or when working with very large Git repos.
• Asynchronous method improvements: subst-async has been tweaked to provide more stability and speed on all systems. usr1 has been made faster through the elimination of a subshell. It is now the default asynchronous method in all Windows environments.
• I have provided the code for my own "Zenburn" custom prompt.