Update flexmark from 0.42.14 to 0.64.8, minSdk to 21

[guanglinn]

1. Update flexmark from 0.42.14 to 0.64.8;
2. Update other libraries;
3. Update minSdk from 18 to 21.

[guanglinn]

Since version 0.42.14, flexmark-java has obtained a large number of improvements and bug fixes, so it is worth upgrading.

Markor works well, and no issues occurred yet after updating flexmark and other libraries.

browser 1.8.0 requires minSdkVersion cannot be smaller than version 19.
appcompat 1.7.0 requires minSdkVersion cannot be smaller than version 21.

Even if we choose API 21 (Android 5.0, in 2014) as minimum SDK, Markor can still run on approximately 99% of devices, so I don't think we need to worry about this.