Add support for avrdude -Pavrdoper

[ajd4096]

https://code.google.com/p/arduino/issues/detail?id=864

[ajd4096]

(forgot the test)
Turn on verbose output, select "AVR-Doper" as the programmer, and verify the avrdude commandline has "-Pavrdoper".

[ajd4096]

2nd commit is a generic external uploader.

I use it mostly with bootloadHID.exe, but you can do almost anything with it.
(eg start cygwin's sh.exe with a script name)

Test using any board, bare minimum sketch, and "Upload using Programmer".

[ffissore]

You can achieve the same result taking advantage of the current boards/platforms definitions (txt files)

Create a custom core with a custom board (for example: a copy of the Arduino Leonardo called "my leonardo") and replace avrdude with a custom uploader. Create a new platform.txt (copying arduino one and change command line params

Select "my leonardo" from the Boards list and upload a sketch