I want to get percentage difference between a quarter compare to the same quarter a year earlier.
This is my `df`

```
df <- data.frame(
  x0 = c('2010Q1', '2010Q2', '2010Q3', '2010Q4', '2011Q1', '2011Q2', 
         '2011Q3', '2011Q4', '2012Q1', '2012Q2', '2012Q3', '2012Q4', 
         '2013Q1', '2013Q2', '2013Q3', '2013Q4', '2014Q1', '2014Q2'),
  x1 = c(14.0, 13.4, 13.8, 14.4, 14.2, 14.3, 14.0, 14.1, 14.6, 14.3, 
         14.0, 13.6, 13.5, 12.9, 13.2, 13.2,12.7, 13.6),
  x2 = c(13.0, 13.3, 13.4, 13.7, 13.7, 13.9, 14.0, 13.9, 13.9, 14.0, 
         14.1, 13.8, 13.7, 13.8, 13.8, 13.8, 13.6, 13.9)
)

```

I want to calculate `2012Q1 / 2011Q1 minus 1`, and for the rest of quarters. to get a `df` like this below: 

```
df <- data.frame(
   x0 = c('2010Q1', '2010Q2', '2010Q3', '2010Q4', '2011Q1', '2011Q2', 
          '2011Q3', '2011Q4', '2012Q1', '2012Q2', '2012Q3', '2012Q4', 
          '2013Q1', '2013Q2', '2013Q3', '2013Q4', '2014Q1', '2014Q2'),
   x1 = c(14.0, 13.4, 13.8, 14.4, 14.2, 14.3, 14.0, 14.1, 14.6, 14.3, 
          14.0, 13.6, 13.5, 12.9, 13.2, 13.2,12.7, 13.6),
   x1_div = c(NA, NA, NA, NA, 0.018, 0.063, 0.009, -0.015, 0.031, 
              0.002, 0.004, -0.036, -0.081, -0.099, -0.059, -0.031, 
             -0.054, 0.057),
   x2 = c(13.0, 13.3, 13.4, 13.7, 13.7, 13.9, 14.0, 13.9, 13.9, 14.0, 
          14.1, 13.8, 13.7, 13.8, 13.8, 13.8, 13.6, 13.9),
   x2_div = c(NA, NA, NA, NA, 0.058, 0.051, 0.044, 0.013, 0.008, 0.006, 
              0.004, -0.008, -0.012, -0.017, -0.016, -0.005, -0.005, 
              0.007)
)

```