Fix #2015 (#2030)

* Added [BOM] (byte-order-mark) text in file encoding info

Like in vim-airline
See https://github.com/vim-airline/vim-airline/pull/992 and fdb74f549d

Ref #2015

* Test with no text added, to check travis results

* make bom a sepaate segment

powerline/config_files/colorschemes/vim/__main__.json

@@ -5,6 +5,7 @@
 		"file_size":          "information:unimportant",
 		"file_format":        "information:unimportant",
 		"file_encoding":      "file_format",
+		"file_bom":         "file_format",
 		"file_type":          "file_format",
 		"branch":             "information:additional",
 		"file_scheme":        "file_name",

powerline/segments/vim/__init__.py

@@ -352,6 +352,18 @@ def file_encoding(pl, segment_info):
 	return vim_getbufoption(segment_info, 'fileencoding') or None
 
 
+@requires_segment_info
+@add_divider_highlight_group('background:divider')
+def file_bom(pl, segment_info):
+	'''Return BOM of the current file
+
+	:return: Byte order mark or None if unknown or missing BOM
+
+	Divider highlight group used: ``background:divider``.
+	'''
+	return 'bom' if vim_getbufoption(segment_info, 'bomb') else None
+
+
 @requires_segment_info
 @add_divider_highlight_group('background:divider')
 def file_type(pl, segment_info):