Add substitution with options

[srebhan]

This PR introduces the SubstituteWithOptions() function including options for custom patterns, custom substitution functions and a new replacement function for allowing to customize the replacement behavior if e.g. no mapping was found.
The existing SubstituteWith function is switched over to the newly introduced function.

[ndeloof]

as there's no issue associated with this PR, can you please clarify how you expect this to be used ? Any upstream project relying on this ?

[srebhan]

@ndeloof we are using this library for Telegraf and have to reproduce a specific behavior namely to only substitute environment-variable expressions if those environment variables are defined. This requirement comes from keeping backward compatibility to not break existing configurations. The issue linked above shows how we use this new function in code.

The intension is to be able to better control the replacement process while keeping the nice shell-like interpolation functionality.

Please let me know if I should open a corresponding issue!

[ndeloof]

For legal reasons we require commits to be signed-off, please amend your commit and force push

[glours]

Is it possible to add an unit test checking the correct behavior of SubstituteWithOptions with a replacement function, please?

[glours]

Just a small modification in your test and we're ready to merge the PR