Add support for tolerant substitution

[kalistace]

node-properties is currently throwing an error when it cannot find a variable.

Sometimes, we want to keep the placeholders as the application itself or another tool will resolve the placeholders further ahead in the process.

The tolerant_substitution option allows this behavior by partially resolving the properties if it cannot find some properties.

[gagle]

Let me check the changes tomorrow and I'll push a new version of this module!

[kalistace]

Thank you. Let me know if you have any questions or if you want me to change something