More configurable separators in NormalizeSeparators

[bhirsz]

Allow to configure how some separators behaves, for example FOR:

FOR    ${var}    ${var2}    IN ZIP    @{list1}    @{list2}

END

uses 4 spaces by default for FOR arguments where it could use 2. Same goes for keyword arguments, tags etc. It could be possible to configure some separators to create more concise output.

Refer to RF coding style guide for inspiration what could be configurable.