backslash in cmd literal escapes newline

[simeonschaub]

As brought up by @ExpandingMan and @giordano on Slack, the current behavior of \ before a newline in cmd literals is a bit odd:

julia> dump(`foo \
       bar`)
Cmd
  exec: Array{String}((2,))
    1: String "foo"
    2: String "\nbar"
  ignorestatus: Bool false
  flags: UInt32 0x00000000
  env: Nothing nothing
  dir: String ""

# note the whitespace before `bar`
julia> dump(`foo \
           bar`)
Cmd
  exec: Array{String}((3,))
    1: String "foo"
    2: String "\n"
    3: String "bar"
  ignorestatus: Bool false
  flags: UInt32 0x00000000
  env: Nothing nothing
  dir: String ""

julia> dump(```
       foo \
       bar```)
Cmd
  exec: Array{String}((2,))
    1: String "foo"
    2: String "\nbar"
  ignorestatus: Bool false
  flags: UInt32 0x00000000
  env: Nothing nothing
  dir: String ""

We should probably just treat \ as a line continuation character and just ignore it.