Conditional styled feature flags

[Rasmus-Bertell]

In my opinion feature flags, styled as conditional statements, would be an improvement. It would allow you to have mutually exclusive lines and cleaner syntax. This would be a breaking change for the earlier implementation of feature flags, but it would make them easier to read without prior knowledge of the correct syntax. Of course, it would be possible to keep the old behavior, but that would make the code more convoluted.

For example, taking the code from the readme:

This line will always be included
### IF FEATURE1
This line will only be included when FEATURE1 is enabled.
### ELSE
This line will be included otherwise.
### END FEATURE1
This line will always be included.

I would be happy to make an effort on implementing this, but I would like to hear your thoughts first.
This could also be expanded to feature logical operators AND, OR, and XOR, but I think those are out of scope for this issue.

[kesslern]

As long as existing templates don't break, I think this would be a great feature.

If it's not too convoluted to keep the old behavior that'd be good... I'm not sure how bad that would be. Maybe use something like ###~ for feature flags with logical operators? I'd like to avoid having breaking changes to dotfiles if at all possible.