Add --safe option to set safe mode level to safe

[ggrossetie]

Asciidoctor Ruby is using the rule "last one wins" when both --safe-mode and --safe options are present:

$ echo '{safe-mode-name}' | asciidoctor -d inline --safe --safe-mode server -
server
$ echo '{safe-mode-name}' | asciidoctor -d inline --safe-mode server --safe -
safe

Or when the options are present multiple times:

$ echo '{safe-mode-name}' | asciidoctor -d inline --safe-mode server --safe --safe-mode unsafe -
unsafe

yargs also allows to use a value when an option is declared as a boolean, so the following commands are valid:

$ asciidoctor --safe=false doc.adoc
$ asciidoctor --safe=true doc.adoc
$ asciidoctor --safe value doc.adoc

I've opened two feature requests on yargs to be able to define flags (ie. boolean without argument) and shortcuts:

• Feature: Declare shortcut option yargs/yargs#1531
• Feature: Declare flag (ie. boolean without argument) yargs/yargs#1532