0.3.4

Added user callback file func.
Fixed a stat bug with obj id.
SPIFFS_probe_fs added
Add possibility to compile a read-only version of spiffs
Make magic dependent on fs length, if needed (see #59 & #66) (thanks @hreintke)
Exposed SPIFFS_open_by_page_function
Zero-size file cannot be seek #57 (thanks @lishen2)
Add tell and eof functions #54 (thanks @raburton)
Make api string params const #53 (thanks @raburton)
Preserve user_data during mount() #51 (thanks @rojer)

New API functions:
SPIFFS_set_file_callback_func
SPIFFS_probe_fs
SPIFFS_open_by_page
SPIFFS_eof
SPIFFS_tell
Added configs:
SPIFFS_READ_ONLY
SPIFFS_USE_MAGIC_LENGTH