Add supertrend indicator

[mario4tier]

@chadfurman when time will come... it seems fairly easy to implement.

https://www.tradingview.com/support/solutions/43000634738-supertrend

TODO before starting: find a reference to validate against.

Details

• Start with cut&paste&modify of ATR interface from table_a.c into table_s.c
• Do gen_code a first time (to create ta_SUPERTREND.c)
• Copy ta_ATR.c "logic" into ta_SUPERTREND.c
• Modify table_s.c for one more input (the multiplier), two output bands and one trend direction array (<0 when downtrend, >0 when uptrend).
• Do gen_code again.
• Add the logic specific to Supertrend.
• Add regression tests (I can do that part, or use again ATR as a model).