coef_plot: global methods to get/set MCMC credible mass

- Inner/outer credible masses for coef_plot, and
coef_catseye were hardcoded at 68%, 95%
- Changed defaults to 50% and 95%, following examples
from Gelman 2007/2013
- Defaults accessible/changeable via
[set_][inner|outer]CM
- Point estimates are now medians (used to be means)
- Added examples for coef_plot_mcmc and coef_catseye
- Minor code formatting changes