Added Neomatrix support, control of file displayed, teensy 3.[56] sdcard, and display tweaks

[marcmerlin]

• added Neomatrix support as alternative to Smartmatrix
• Added teensy 3.5/3.6 built in sdcard support
• Added option to show decoded values of a single line of a single GIF for debugging
• while(1) will crash ESPxx chips, replaced with long delay
• added support for showing specific file by index number, or change files with n/p over serial port
• removed spurious newlines in filenamefunctions and double display of filenames
• added support for changing gamma value on NEOMATRIX backend
• allow staying on a single GIF for 1h instead of 10 seconds for debugging/gamma adjustement and long GIFS

[marcmerlin]

For anyone reading this PR, you'll probably want to head directly to https://github.com/marcmerlin/AnimatedGIF for a more up to date version with more hardware support (ESP8266 and ESP32 SPIFFS, plus easy to use NeoMatrix)