Consider adding support for Flyway 'Placeholder replacement' feature

[srcshelton]

myway supports marker-substitution for Stored Procedures.

Flyway has a more generic placeholder replacement system for terms in the form of ’${placeholder}`.

In many ways, this feature raises more questions than it answers - since changing the value substituted for a given placeholder could significantly change the action taken as a result, this value would need to be stored and validated. Should it be considered an error to re-run a migration with a different substitution against the same database? Should a migration where a placeholder value is not specified inherit the previous value? Do we now have to pre-process all migrations to ensure that all placeholders have provided values?

The stated Flyway use-case for this feature is to perform different actions in different environments - something which myway achieves in an arguably cleaner fashion via the 'environment' metadata directive.

[srcshelton]

The relevant Flyway options here are:

• placeholderPrefix, default '${'
• placeholderSuffix, default '}'
• placeholders.<name>=<value>, noting that 'placeholder' is now plural

... it is unclear how Flyway handles a placeholder whose name (or value) includes an '=' character...