[merge-conf] Some minor modifications/additions to merge-conf

I have attached 3 patches with some minor modifications to merge-conf:

1) Adds support for meld as well as vimdiff for external merging.  Pretty self-explanatory.

2) Changes the order that files are passed to the external merge tool. When I merge rpm changes, I generally merge left to right.  So, when merging an ".rpmnew", left contains the ".rpmnew" and right contains the final file. When merging an ".rpmsave", left contains the .rpmsave, and right contains the final file. In all cases, final file (i.e. new) is on the right and "old" file is on the left. I think this makes more sense than the current order.

3) Currently, if vimdiff or meld are used to do the merge, the .rpmnew or .rpmsave file is not deleted. This last patch adds a user prompt to delete this file.

Here is the diffstat:

 plugins/merge-conf/merge-conf.py |   43 ++++++++++++++++++++++++++++++--------
 1 files changed, 34 insertions(+), 9 deletions(-)

Cheers,
Raman