[TEMPLATE.sh.tpl] Switch to using upper-case names for global variables:

- Makes it easier to differntiate local variables in functions.
- Allows for avoiding "shadowing" of lower-cased/upper-cased variable
names by mistake in functions.

Noticket: [TEMPLATE.sh.tpl] Improve help output and tweak quiet/debug option processing:

- Introduce short and long help output. Short help includes only the
basic usage information, while long help provides full description
with all options etc.
- Short help includes only the basic usage, and should fit much more
easily on a single screen of output.
- Long help is only shown when script is invoked with the -h option.
- Output warning messages to stderr instead of stdout.
- Hide info and success messages if quiet output is requested.
- Introduce check for exclusiveness between the debug and quiet
options.
- Fix some smaller issues reported by shellcheck.

Noticket: [TEMPLATE.sh.tpl] Make sure to default the value for $1 positional argument when checking if it has been passed-in:

- Otherwise the shell script will error-out because the variable is
not defined.

Noticket: [TEMPLATE.sh.tpl] Update snippet syntax for latest(ish) YASnippet:

- Use correct syntax for dynamic values.
- Perform the necessary escaping for the YASnippet syntax.
- Introduces quiet and debug mode options.

Noticket: [TEMPLATE.sh.tpl] Fix a minor issue reported by shellcheck:

- Do not use $/${} on arithmetic variables (not needed).