[enh] Add timeout limit per request (#1640)

The new url parameter "timeout_limit" set timeout limit defined in second.
Example "timeout_limit=1.5" means the timeout limit is 1.5 seconds.

In addition, the query can start with <[number] to set the timeout limit.

For number between 0 and 99, the unit is the second :
Example: "<30 searx" means the timeout limit is 3 seconds

For number above 100, the unit is the millisecond:
Example: "<850 searx" means the timeout is 850 milliseconds.

In addition, there is a new optional setting: outgoing.max_request_timeout.
If not set, the user timeout can't go above searx configuration (as before: the max timeout of selected engine for a query).

If the value is set, the user can set a timeout between 0 and max_request_timeout using
<[number] or timeout_limit query parameter.

Related to #1077
Updated version of PR #1413 from @isj-privacore

searx/query.py

@@ -43,6 +43,7 @@ class RawTextQuery(object):
         self.query_parts = []
         self.engines = []
         self.languages = []
+        self.timeout_limit = None
         self.specific = False
 
     # parse query, if tags are set, which
@@ -69,6 +70,21 @@ class RawTextQuery(object):
                 self.query_parts.append(query_part)
                 continue
 
+            # this force the timeout
+            if query_part[0] == '<':
+                try:
+                    raw_timeout_limit = int(query_part[1:])
+                    if raw_timeout_limit < 100:
+                        # below 100, the unit is the second ( <3 = 3 seconds timeout )
+                        self.timeout_limit = float(raw_timeout_limit)
+                    else:
+                        # 100 or above, the unit is the millisecond ( <850 = 850 milliseconds timeout )
+                        self.timeout_limit = raw_timeout_limit / 1000.0
+                    parse_next = True
+                except ValueError:
+                    # error not reported to the user
+                    pass
+
             # this force a language
             if query_part[0] == ':':
                 lang = query_part[1:].lower().replace('_', '-')
@@ -161,7 +177,7 @@ class RawTextQuery(object):
 class SearchQuery(object):
     """container for all the search parameters (query, language, etc...)"""
 
-    def __init__(self, query, engines, categories, lang, safesearch, pageno, time_range):
+    def __init__(self, query, engines, categories, lang, safesearch, pageno, time_range, timeout_limit=None):
         self.query = query.encode('utf-8')
         self.engines = engines
         self.categories = categories
@@ -169,6 +185,7 @@ class SearchQuery(object):
         self.safesearch = safesearch
         self.pageno = pageno
         self.time_range = time_range
+        self.timeout_limit = timeout_limit
 
     def __str__(self):
         return str(self.query) + ";" + str(self.engines)