Allowing flashing mode to be selected by the user

Greetings developers & contributors.

LoLin NodeMCU V3 is a widely available, Cheapest ESP8266 board and uses 26MHz crystal(Even on latest boards) and so the flashing mode needs to be set to `0x0040`. As it's being done in the esptool.

Without this, flashing using esp-web-tools results in reset loop as the header patching was removed in esp-web-flasher https://github.com/NabuCasa/esp-web-flasher/pull/86 as it was perceived to be useless for ESP32 boards.

The release note for the change states,

> It's up to the caller of the library to provide the right values

So in this case esp-web-tools is the caller and I think the best way to address this is for allowing the user to select different flashing mode via a setting in the manifest.