Add SPIFFS.check() to implement checking capabilities.

[miky2k]

In sensors log with casual power loss may happen that file-system stop working (especially remove()),
there is a check implementation in SPIFFS library that should exposed in Arduino.

[ABH10]

I would appreciate this enhancement very much. Once in a while SPIFFS seems to mess up for no obvious reason. Currently an SPIFFS.format() is the only cure.