Added deceleration feature

[femtotech]

Added a setting for deceleration. You can now set a different value for acceleration and deceleration.

[pkerspe]

would love to see this feature merged @Stan-Reifel since I am also using flexy stepper in a bigger project, where an emergency stop is currently implemented in a more "nasty" way due to the lack of this function in the library

[pkerspe]

@femtotech in case you are interested: I created a new library based on this one but as an independent library (mostly focusing on ESP modules like the ESP32 / ESP 8266) and incorporated your pull request's changes.
I have the feeling this library here is no longer maintained, thus my decision to create a new one rather than a fork: https://github.com/pkerspe/ESP-FlexyStepper

[cranefist]

I tested this but it does not work well at all, confuses steps completely if target step is changed while in motion few times.

It would also need a way to change the deceleration distance to be useful.

EDIT. I think my issues came from changing acceleration & deceleration while in movement. This would also be a useful function. Because depending on how long movements i'm making, i need to adjust the acceleration to avoid jerky small movements.