Add option 'm' for trimming whitespaces in snippet

Fix #391

doc/UltiSnips.txt

@@ -649,6 +649,11 @@ The options currently supported are: >
        before jumping to the next tabstop.  This is useful if there is a
        tabstop with optional text at the end of a line.
 
+   m   Trim all whitespaces from right side of snippet lines. Practically
+       useful when snippet contains empty lines which should remain empty
+       after expanding on some indentation level. Without this option
+       empty lines in snippets definition will have indentation too.
+
 The end line is the 'endsnippet' keyword on a line by itself. >
 
    endsnippet

pythonx/UltiSnips/snippet/definition/_base.py

@@ -228,7 +228,10 @@ class SnippetDefinition(object):
             if line_num != 0:
                 line_ind = indent + line_ind
 
-            initial_text.append(line_ind + line[tabs:])
+            result_line = line_ind + line[tabs:]
+            if 'm' in self._opts:
+                result_line = result_line.rstrip()
+            initial_text.append(result_line)
         initial_text = '\n'.join(initial_text)
 
         snippet_instance = SnippetInstance(

test/test_Chars.py

@@ -47,6 +47,16 @@ class RemoveTrailingWhitespace(_VimTest):
     wanted = """Hello\nGoodbye"""
     keys = 'test' + EX + BS + JF + 'Goodbye'
 
+class TrimSpacesAtEndOfLines(_VimTest):
+    snippets = ('test', """next line\n\nshould be empty""", '', 'm')
+    wanted = """\tnext line\n\n\tshould be empty"""
+    keys = '\ttest' + EX
+
+class DoNotTrimSpacesAtEndOfLinesByDefault(_VimTest):
+    snippets = ('test', """next line\n\nshould be empty""", '', '')
+    wanted = """\tnext line\n\t\n\tshould be empty"""
+    keys = '\ttest' + EX
+
 
 class LeaveTrailingWhitespace(_VimTest):
     snippets = ('test', """Hello \t ${1:default}\n$2""")