Update compile flag to c++14 (or c++17)

**Is your feature request related to a problem? Please describe.**
I want to add a c++ library that depends on C++14 features. Upgrading from c++11 to c++14 should take care of that.

**Describe the solution you'd like**
Update the platform.txt file to update the gcc command like flags.
https://github.com/adafruit/Adafruit_nRF52_Arduino/blob/master/platform.txt#L43

**Describe alternatives you've considered**
I have tested it here locally on a fork and haven't seen any issues (c++14 should be backward compatible with c++11).

**Additional context**
If you are OK with going ahead, I can make the change here and create a PR. I just want to know if you would approve the update as it could impact other things I may not be aware of. The gcc arm compiler has been supporting this flag for quite a while.

Start with platform.txt around line 43 and inspect the GCC command-like flags used for compilation. Confirm whether the requested C++14 or C++17 setting is appropriate, then verify that the affected Arduino builds still compile successfully with the updated flag.

Written by the indexing model from the issue text.