Allow adding or removing individual dependencies.

Specifying a DependOnService or DependOnGroup argument as +<dependency>
will add the dependency to the list.  Specifying it as -<dependency>
will remove it from the list.  The dependency may also be given as
:<dependency> to make scripting more legible.

Groups should technically be prefixed with a + symbol.  NSSM will follow
the rules above even when the + prefix is used.