release v2.1.3

Full Changelog: v2.1.2...v2.1.3

v2.1.1

• fixed: Class File add a operator =
• reduce compile warning.
• remove K210 support.
• reorganize the library structure
• remove SFUD, It shouldn't be here
• remove more k210 relate code
• update .travis.yml
• update readme.
• make read return size_t
• fixed: Overloaded "=" operator needs to return * this
• support wio lite ai sdmmc.
• update examples

v2.0.3

• Fixed incorrect constant being used to test result from f_read( ), reversing logic as needed
• add ~File(), close file while ~File()

v2.0.2

• Updated library.properties information

v2.0.1

• Added const qualifier to File::operator bool()
• Updated library.properties to v2.0.1

v2.0.0

• fix: default mode to SD
• fix: write function error in the flash

v1.1.1

Update issue templates

support POSIX mode flag

Merge pull request #1 from Seeed-Studio/beta

support POSIX opration

v1.0.0

Update README.md