add a data argument to leaflet()
avoid evaluating the arguments specified in `excludes`
adjust the examples after changes in #3
renamed appendList() to appendMapData(); added a new function evaluateFormula() to evaluate list members that are formulae using the data as the environment if possible
an example of using the formula interface
roxygenize
use the formula environment as the enclosing environment of eval()
add an example in which radius is not from the data
more name changes: addTiles() and addCircleMarkers()
recursively evaluate the options list if any members are formula
make sure the formula is one-sided
fixed the wrong recursion
add an example of using the color option as a formula
Merge pull request #5 from rstudio/feature/formula-interface

The formula interface for layer functions