Fix the column modifications on escaped separators

If a g:table_mode_separator is escaped by a backslash, the table is
aligned ignoring this instance of the g:table_mode_separator, but the
column deletion and insertion functions did not consider this behaviour.

The g:table_mode_escaped_separator regex is simplified and optimized a
little bit. Also more care is taken to prevent unexpected behaviour if
special characters are used as g:table_mode_separator.

t/config/options.vim

@@ -1,6 +1,6 @@
 let g:table_mode_corner = '+'
 let g:table_mode_separator = '|'
-let g:table_mode_escaped_separator_regex = '\(\\\)\@<!|'
+let g:table_mode_escaped_separator_regex = '\V\C\\\@1<!|'
 let g:table_mode_fillchar = '-'
 let g:table_mode_header_fillchar = '-'
 let g:table_mode_map_prefix = '<Leader>t'

t/fixtures/escaped_seperator.txt

@@ -0,0 +1,9 @@
+|-------------------+---------------------------|
+| The \| works as   | It can be escaped by a \. |
+| a separator.      |                           |
+|                   |                           |
+| Escaping \ with   | This feature would        |
+| a \ doesn't work. | be unnecessary, because   |
+|                   | a separator must be       |
+|                   | preceded by a space.      |
+|-------------------+---------------------------|

t/spreadsheet.vim

@@ -226,4 +226,34 @@ describe 'spreadsheet'
       Expect tablemode#spreadsheet#ColumnCount('.') == 4
     end
   end
+
+  describe 'Escaped table separators'
+    before
+      new
+      normal! ggdG
+      read t/fixtures/escaped_seperator.txt
+      call cursor(2, 3)
+    end
+
+    it 'should not prevent the deletion of rows'
+      Expect tablemode#spreadsheet#RowCount('.') == 7
+      call tablemode#spreadsheet#DeleteRow()
+      Expect tablemode#spreadsheet#RowCount('.') == 6
+      Expect getline('.') == '| a separator.      |                         |'
+    end
+
+    it 'should not prevent the deletion of columns'
+      Expect tablemode#spreadsheet#ColumnCount('.') == 2
+      call tablemode#spreadsheet#DeleteColumn()
+      Expect tablemode#spreadsheet#ColumnCount('.') == 1
+      Expect getline('.') == '| It can be escaped by a \. |'
+    end
+
+    it 'should not prevent the insertion of columns'
+      Expect tablemode#spreadsheet#ColumnCount('.') == 2
+      call tablemode#spreadsheet#InsertColumn(1)
+      Expect tablemode#spreadsheet#ColumnCount('.') == 3
+      Expect getline('.') == '| The \| works as   |  | It can be escaped by a \. |'
+    end
+  end
 end