Include a Staging option

[chalkers]

We should be able to test binaries and runtimes in a "staging" environment.

There could be a preferences window or a menu item that would switch this on and off.

[codealchemist]

What if we add an option to the version dropdown ("Select binaries to flash") called "Custom" or something like that to display needed fields for a custom build.

We will fill something like this:

• URL: http://www.espruino.com/files/espruino_1v96.zip
• Address: 0x0000
• Path: espruino_1v96_esp8266_4mb/boot_v1.6.bin

What do you think?