Prevent shuffling with mergemode prepend or append

[badgerious]

As noted in the README, if multiple puppet resources manage the same variable and are all in prepend/append mode, each will promote its value to the beginning/end when it is evaluated (this will happen on every puppet run). This can be solved by using an array of values in a single resource, but this doesn't work well for multiple third party modules that want to prepend/append independently.

Possible solutions:

1. A prependonce and appendonce option that will prepend/append if the value does not appear, but will leave the value where it is if it does. Leave prepend and append as is for backward compatibility.
2. A priority option that allows an order to be specified among the resources (a la puppet firewall rules). Same priority resources would be allowed in any order. Some potentially odd edge cases (value => 'x', priority => 2 for one resource and value => ['x', 'y'], priority => 1 for another).

To anyone who uses this module and cares about this, please comment. I'll not make the time to look into this unless I hear feedback.