Implement mechanism to replace a block instead of rewriting the file

[pinpox]

Some of the base16-supported applications have no mechanism to source other configuration files from the main config, e.g. applications that are configured via YAML. For the apps that don't allow for a source or include statement in their configuration it would be nice to be able to place a specially marked block in their config that is replaced with the newly generated content instead of rewriting the whole file

e.g. for a YAML file:

value1: foo
value2: bar

# BEGIN BASE16 MANAGED BLOCK
color1: 333333
color2: 444444
# END BASE16 MANAGED BLOCK

value3: baz

The mechanism should then just substitute the marked block