PostingsSolrHighlighter support for CustomSeparatorBreakIterator (LUCENE-6485)

Lucene 5.3 added a CustomSeparatorBreakIterator (see LUCENE-6485)

SOLR-10152.patch uploaded which incorporates CustomSeparatorBreakIterator in PostingsSolrHighlighter.

• added a new request param option to specify which separator char to use. customSeparatorChar.
• changed PostingsSolrHighlighter.getBreakIterator to check HighlightParams.BS_TYPE first.
• if type=='CUSTOM', look for the new separator param, in getBreakIterator, validate it's a single char, & skip locale parsing.
• 'WHOLE' option moved from parseBreakIterator to getBreakIterator, as it doesn't depend on locale.

Changes made in:

• HighlightParams.java
• PostingsSolrHighlighter.java
• test cases added in TestPostingsSolrHighlighter