Support skip ranges

[jmbarbone]

Probably related to #114

I'd appreciate some general, inline format skipping options.

If I wanted to do something like:

one   <- c(TRUE, TRUE,  TRUE )
two   <- c(TRUE, FALSE, NA   )
three <- c(TRUE, TRUE,  FALSE)

current workarounds

multiple skips

# fmt: skip 
one   <- c(TRUE, TRUE,  TRUE )
# fmt: skip 
two   <- c(TRUE, FALSE, NA   )
# fmt: skip 
three <- c(TRUE, TRUE,  FALSE)

getting by with {

# fmt: skip 
{
  one   <- c(TRUE, TRUE,  TRUE )
  two   <- c(TRUE, FALSE, NA   )
  three <- c(TRUE, TRUE,  FALSE)
}

preferred solutions

start:end keywords

# fmt: skip start
one   <- c(TRUE, TRUE,  TRUE )
two   <- c(TRUE, FALSE, NA   )
three <- c(TRUE, TRUE,  FALSE)
# fmt: skip end

number of lines/expressions (fmt: skip -> fmt: skip 1)

# fmt: skip 3
one   <- c(TRUE, TRUE,  TRUE )
two   <- c(TRUE, FALSE, NA   )
three <- c(TRUE, TRUE,  FALSE)

[lionel-]

I think I prefer the start/end style because it's easier to edit.

One necessary invariant for this sort of comments would be that they must be direct children of either the program list or a brace list.