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 some dependency libraries;
3. Update minSdk from 18 to 21.

[guanglinn]

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

Marder 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 we don't need to worry too much.

[gsantner]

For sure there are improvements at every release. Though is there something specific that gave you reason to get a newer version?

[JSchmerling]

From what I've seen so far, the benefits of upgrading are improved performance (https://github.com/vsch/flexmark-java/wiki/Version-0.60.0-Changes) and better handling of MarkDown extensions (flexmark-all).

[gsantner]

I think the time is now right to update to a newer minimum SDK version. Let's merge this after the next update.

[guanglinn]

I will implement rich text pasting after this PR merged.