Support skip ranges

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)

Start by reviewing issue #114 and the existing fmt: skip behavior described here. Compare the proposed start/end and line-count forms, then establish the accepted syntax and verify that range skipping works without repeated directives.