Slight changes to template handling

[joshrotenberg]

Some minor changes to the new template feature:

1. Changed the environment variable option to be ADRS_TEMPLATE instead of ADRS_TEMPLATE_DIR since the arg points to the file itself
2. Changed the flag to be lowercase '-t'
3. Changed the behavior to be the following:
   • if a file is explicitly specified via the flag or ADRS_TEMPLATE, that file is used as the template
   • if no flag or env is specified, use $adr_dir/templates/template.md
   • if no #adr_dir/templates/template.md exists, use the built in default template

Fixes #47.

@lbenezriravin Let me know what you think about these changes when you have time.

[joshrotenberg]

Going for it ...