* emon: configure ratios without reboot
* settings: serialize() support
* debug: use vsnprintf from newlib, not from sdk
* settings/experimental: show defaults via `get`
* emon: override base methods, fix defaults
* sensor/emon: expose internal index calculation
- refactor configuration to use the correct index when accessing indexed
sensor methods. store index value on magnitude, refactor loops to
accomodate this new functionality
- rename slot(index) -> description(index), since we use 'slot' as
numeric value
- general conversion from .ino modules into a separate .cpp files
- clean-up internal headers, place libraries into .h. guard .cpp with _SUPPORT flags
- fix some instances of shared variables instead of public methods
- tweak build system to still build a single source file via os environment variable ESPURNA_BUILD_SINGLE_SOURCE
