enhancement/extra-hl-fields (#92)

As noted in issue#65 by @aaccomazzi, there are 3 more fields available
when using highlights:
 * ack
 * aff
 * author

Updated keyword and README

README.md

@@ -115,7 +115,7 @@ The ADS allows you to search content that was extracted from the abstract, body,
 {'abstract': [' in the early universe or in the ultra-dense core of neutron <em>stars.</em> The thermal radiation from the quarks']}
 ```
 
-The tags **<em></em>** *highlight* the relevant text. This feature is only available when searching the fields `title, abstract, body`.
+The tags **<em></em>** *highlight* the relevant text. This feature is only available when searching the fields `title`, `abstract`, `body`, `ack`, `aff`, `author`.
 
 
 **Rate limits and optimisations**

ads/search.py

@@ -345,7 +345,7 @@ class SearchQuery(BaseQuery):
     HTTP_ENDPOINT = SEARCH_URL
     DEFAULT_FIELDS = ["author", "first_author", "bibcode", "id", "year",
                       "title"]
-    HIGHLIGHT_FIELDS = ["abstract", "title", "body"]
+    HIGHLIGHT_FIELDS = ["abstract", "title", "body", "ack", "aff", "author"]
 
     def __init__(self, query_dict=None, q=None, fq=None, fl=DEFAULT_FIELDS,
                  sort=None, cursorMark=None, start=None, rows=50, max_pages=1,