Dealing with y1/y2 ~ x formulas

[zeileis]

Fixes #339

After the discussion in the issue, I tried to address this with two improvements:

1. Explicit pointer to I() in the warning message.
2. More details in the documentation of the formula argument.

Example for 1:

tinyplot(mpg/hp ~ wt, data = mtcars)
## Warning message:
## In tinyplot.formula(mpg/hp ~ wt, data = mtcars) :
##   formula should specify at most one y-variable, using: mpg
## if you want to use arithmetic operators, make sure to wrap them inside I()

From the ?tinyplot documentation for argument formula:

a formula that optionally includes grouping variable(s) after a vertical bar, e.g. y ~ x | z. One-sided formulae are also permitted, e.g. ~ y | z. Only a single y and x variable (if any) must be specified but multiple grouping variables can be included in different ways, e.g. y ~ x | z1:z2 or y ~ x | z1 + z2. (These two representations are treated as equivalent; both are parsed as interaction(z1, z2) internally.) If arithmetic operators are used for transforming variables, they should be wrapped in I(), e.g., I(y1/y2) ~ x. Note that the formula and x arguments should not be specified in the same call.

[grantmcdermott]

lgtm!