custom values for k in ema function

[Zahari Zankov]

Would you be open to allowing the ema function handle custom kvalues?

I can see your latest implementation handles Wells Wilder type EMA which is great and will hopefully land CRAN soon. Having an even less constraint implementation would allow easier building of indicators like Price Momentum Oscillator with k = 2 / n.

I would happily pitch in too if interested.