main: add -programmer flag

This flag is overloaded. It can be used in two ways:

  * Choosing the flash method to use (openocd, msd, command).
  * Choosing the OpenOCD programmer name.

For example, you can use one of these to use OpenOCD instead of the
mass-storage device programmer:

    tinygo flash -target=microbit -programmer=openocd
    tinygo flash -target=microbit -programmer=cmsis-dap

compileopts/options.go

@@ -19,4 +19,5 @@ type Options struct {
 	WasmAbi       string
 	HeapSize      int64
 	TestConfig    TestConfig
+	Programmer    string
 }