ENH: inconsistent naming convention for read_csv and read_excel column selection
`read_csv` uses the `usecols` parameter, and the `read_excel` function uses `parse_cols` parameter. Both parameter do basically the same thing:

```
usecols : array-like
    Return a subset of the columns.
    Results in much faster parsing time and lower memory usage.
```

```
parse_cols : int or list, default None
    * If None then parse all columns,
    * If int then indicates last column to be parsed
    * If list of ints then indicates list of column numbers to be parsed
    * If string then indicates comma separated list of column names and
      column ranges (e.g. "A:E" or "A,C,E:F")
```

Also maybe supporting the same thing that `parse_cols` does in `usecol` wouldn't be so bad?
