add prior explorer - tiny shiny app?

definitely a wish list item, not immediate:

write a function that accepts a `specs` list of priors (could even use `manipulate` or Shiny to return an edited list), e.g.:

``` r
GPP_daily_mu 10
GPP_daily_sigma 10
ER_daily_mu -10
ER_daily_sigma 10
K600_daily_mu_mu 30
K600_daily_mu_sigma 30
K600_daily_sigma_shape 1
K600_daily_sigma_rate 0.5
err_obs_iid_sigma_shape 1
err_obs_iid_sigma_rate 10
err_proc_iid_sigma_shape 1
err_proc_iid_sigma_rate 100
```

and visualizes the priors distributions indicates by that `specs` list.

i often forget how and/or don't bother to do this. would be nice to see a set of plots like the following but prettier, informatively labeled, and showing all of the relevant distributions (one plot for `GPP_daily`, one for `K600_daily_mu`, etc.)

``` r
x <- seq(0,5,by=0.1)
plot(x, dgamma(x, shape=1, rate=0.5), type='l', ylim=c(0,1), ylab='dgamma')
lines(x, dgamma(x, shape=1, rate=10), col='blue')
lines(x, dgamma(x, shape=1, rate=100), col='red')
```