tidyselect 0.2.0

The main point of this release is to revert a troublesome behaviour
introduced in tidyselect 0.1.0. It also includes a few features.

Evaluation rules

The special evaluation semantics for selection have been changed
back to the old behaviour because the new rules were causing too
much trouble and confusion. From now on data expressions (symbols
and calls to : and c()) can refer to both registered variables
and to objects from the context.

However the semantics for context expressions (any calls other than
to : and c()) remain the same. Those expressions are evaluated
in the context only and cannot refer to registered variables.

If you're writing functions and refer to contextual objects, it is
still a good idea to avoid data expressions. Since registered
variables are change as a function of user input and you never know
if your local objects might be shadowed by a variable. Consider:

n <- 2
vars_select(letters, 1:n)

Should that select up to the second element of letters or up to
the 14th? Since the variables have precedence in a data expression,
this will select the 14 first letters. This can be made more robust
by turning the data expression into a context expression:

vars_select(letters, seq(1, n))

You can also use quasiquotation since unquoted arguments are
guaranteed to be evaluated without any user data in scope. While
equivalent because of the special rules for context expressions,
this may be clearer to the reader accustomed to tidy eval:

vars_select(letters, seq(1, !! n))

Finally, you may want to be more explicit in the opposite direction.
If you expect a variable to be found in the data but not in the
context, you can use the .data pronoun:

vars_select(names(mtcars), .data$cyl : .data$drat)

New features

• The new select helper last_col() is helpful to select over a
  custom range: vars_select(vars, 3:last_col()).

• : and - now handle strings as well. This makes it easy to
  unquote a column name: (!! name) : last_col() or -(!! name).

• vars_select() gains a .strict argument similar to
  rename_vars(). If set to FALSE, errors about unknown variables
  are ignored.

• vars_select() now treats NULL as empty inputs. This follows a
  trend in the tidyverse tools.

• vars_rename() now handles variable positions (integers or round
  doubles) just like vars_select() (#20).

• vars_rename() is now implemented with the tidy eval framework.
  Like vars_select(), expressions are evaluated without any user
  data in scope. In addition a variable context is now established so
  you can write rename helpers. Those should return a single round
  number or a string (variable position or variable name).

• has_vars() is a predicate that tests whether a variable context
  has been set (#21).

• The selection helpers are now exported in a list
  vars_select_helpers. This is intended for APIs that embed the
  helpers in the evaluation environment.

Fixes

• one_of() argument vars has been renamed to .vars to avoid
  spurious matching.