Allow custom cycle length

[cy]

Thanks for the great app, tried it out for a few months - but the predictions have always been way early (>1week) for me. This is because the app doesn't have enough of my data to calculate a reliable average. Also there was a month when I was terribly sick and my cycle length was much shorter than normal, throwing off the average calculation.

Would you be open to a PR that allows users to set custom cycle length? This will also allow first-time users to get more accurate predictions from the onset, given they know their average length. The calculated average will still be shown, and the user can go back to using the calculated average at anytime.

[jessamynsmith]

That makes sense, I have a pretty regular cycle but I can see if you have some really far-off data it wouldn't work well. If you submit a pull request I will certainly look at it. :)

[cy]

OK! Will do!