Add threshold for "same"

Can adjust `trend_terms()` to have a modifier small differences
```r
headline(20.4, 20.5, "A {trend} of {delta} ({delta_p})%")
# A decrease of 0.1 (0.5%)
```
desired, uses percentage change
```r
headline(20.4, 20.5, "A {trend} of {delta} ({delta_p})%", threshold = 0.03) 
# A small decrease of 0.1 (0.5%) 
# 0.5% is actually 0.005, well below 0.03
```
