Multiline strings support for config

Hi,

enclosed patch should add support for haskell-like multiline strings in xmobar's resource file.
This may also solve older issue #92 .
It replaces former `strField` function so all parsers using it are affected. Most suitable for lengthy template field.

The template string now may be broken in multiple rows (example from .xmobarrc):
```haskell
       , sepChar = "%"
       , alignSep = "}{"
       , template = "%StdinReader% }{ [\
            \<fc=steelblue><action=`mpc -q toggle`>\
            \<action=`mpc -q next` button=3><action=`mpc -q prev` button=2>\
            \%mpd%</action></action></action></fc>\
            \<fc=steelblue2><action=`amixer -q sset Master toggle`>\
            \<action=`amixer -q sset Master 5%%+` button=3>\
            \<action=`amixer -q sset Master 5%%-` button=2>\
            \%default:Master%</action></action></action></fc>|\
            \<fc=limegreen>%wlan0wi%</fc>|%thermal0%|%battery%|\
            \%LKPR% <fc=#ee9a00>%mydate%</fc>|\
            \<fc=yellow><action=`xkb-switch -n`>%kbd%</action></fc>" }
```
It was also necessary to remove case from `stripComments` function when leading unescaped backslash gets removed. Didn't get the reason what it has to do with comments stripping (?).
Row comments starting with `--` are still properly stripped.

Take care