Add support for highlight query

Closes #854

Author: gmarz

src/Nest/DSL/Search/HighlightDescriptor.cs

@@ -46,6 +46,9 @@ public interface IHighlightRequest
 
 		[JsonProperty("boundary_chars")]
 		string BoundaryChars { get; set; }
+
+		[JsonProperty("highlight_query")]
+		IQueryContainer HighlightQuery { get; set; }
 	}
 
 	public class HighlightRequest : IHighlightRequest
@@ -62,6 +65,7 @@ public class HighlightRequest : IHighlightRequest
 		public Dictionary<PropertyPathMarker, IHighlightField> Fields { get; set; }
 		public bool? RequireFieldMatch { get; set; }
 		public string BoundaryChars { get; set; }
+		public IQueryContainer HighlightQuery { get; set; }
 	}
 
 	public class HighlightDescriptor<T> : IHighlightRequest
@@ -93,6 +97,8 @@ public class HighlightDescriptor<T> : IHighlightRequest
 
 		string IHighlightRequest.BoundaryChars { get; set; }
 
+		IQueryContainer IHighlightRequest.HighlightQuery { get; set; }
+
 		public HighlightDescriptor<T> OnFields(params Action<HighlightFieldDescriptor<T>>[] fieldHighlighters)
 		{
 			fieldHighlighters.ThrowIfEmpty("fieldHighlighters");
@@ -182,5 +188,10 @@ public HighlightDescriptor<T> BoundaryMaxSize(int boundaryMaxSize)
 			Self.BoundaryMaxSize = boundaryMaxSize;
 			return this;
 		}
+		public HighlightDescriptor<T> HighlightQuery(Func<QueryDescriptor<T>, QueryContainer> query)
+		{
+			Self.HighlightQuery = query(new QueryDescriptor<T>());
+			return this;
+		}
 	}
 }

src/Tests/Nest.Tests.Integration/Search/HighlightTests.cs

@@ -80,5 +80,53 @@ public void TestHighlightNoNullRef()
 			Assert.GreaterOrEqual(result.Total, 1);
 			Assert.AreEqual(result.Highlights.Count(), 0);
 		}
+
+		[Test]
+		public void TestHighlightQuery()
+		{
+			var result = this.Client.Search<ElasticsearchProject>(s => s
+			  .From(0)
+			  .Size(10)
+			  .Query(q => q
+				.QueryString(qs => qs
+				  .Query("elasticsearch.pm")
+				)
+			  )
+			  .Highlight(h => h
+				.HighlightQuery(hq => hq
+					.Bool(b => b
+						.Must(m => m
+							.Match(mm => mm
+								.OnField(p => p.Name)
+								.Query("elasticsearch.pm")
+							)
+						)
+						.Should(sh => sh
+							.MatchPhrase(mp => mp
+								.OnField(p => p.Name)
+								.Slop(1)
+								.Boost(10)
+							)
+						)
+						.MinimumShouldMatch(0)
+					)
+				)
+				.PreTags("<b>")
+				.PostTags("</b>")
+				.OnFields(
+				  f => f
+					.OnField(e => e.Name)
+					.PreTags("<em>")
+					.PostTags("</em>")
+				)
+			  )
+			);
+
+			Assert.IsTrue(result.IsValid);
+			Assert.DoesNotThrow(() => result.Highlights.Count());
+			Assert.IsNotNull(result.Highlights);
+			Assert.GreaterOrEqual(result.Total, 2);
+			Assert.AreEqual(result.Highlights.Count(), 2);
+		}
 	}
 }

src/Tests/Nest.Tests.Unit/Search/Highlight/HighlightTests.cs

@@ -13,6 +13,12 @@ public void TestHighlight()
 				.From(0)
 				.Size(10)
 				.Highlight(h => h
+					.HighlightQuery(hq => hq
+						.Match(m => m
+							.OnField(p => p.Name)
+							.Query("nest")
+						)
+					)
 					.BoundaryCharacters(".,!? \t\n")
 					.BoundaryMaxSize(20)
 					.Encoder("html")
@@ -52,7 +58,14 @@ public void TestHighlight()
 			  }
 			},
 			require_field_match: true,
-			boundary_chars: "".,!? \t\n""
+			boundary_chars: "".,!? \t\n"",
+			highlight_query: {
+				match: {
+					name: {
+						query: ""nest""
+					}
+				}
+			}
 		  }
 		}";
 			Assert.True(json.JsonEquals(expected), json);