Allow the use of non-local ReadMe files in the CDS reader

Currently, the ReadMe file of a CDS ascii file has to be stored locally to be able to read it. This patch change the opening of the readme file (when used). Now it uses the already implemented *BaseInputter* that can read a wide variety of inputs, including remote URLs.

The input of remote CDS files can be tested with:
```python
from astropy.io import ascii

# Open remote files glob
data_remote = "https://raw.githubusercontent.com/nudomarinero/astropy/master/astropy/io/ascii/tests/t/cds/glob/lmxbrefs.dat"
readme_remote = "https://raw.githubusercontent.com/nudomarinero/astropy/master/astropy/io/ascii/tests/t/cds/glob/ReadMe"
table1 = ascii.read(data_remote, readme=readme_remote)

# Open remote files multi
data_remote_multi1 = "https://raw.githubusercontent.com/nudomarinero/astropy/master/astropy/io/ascii/tests/t/cds/multi/lhs2065.dat"
data_remote_multi2 = "https://raw.githubusercontent.com/nudomarinero/astropy/master/astropy/io/ascii/tests/t/cds/multi/lp944-20.dat"
readme_remote_multi = "https://raw.githubusercontent.com/nudomarinero/astropy/master/astropy/io/ascii/tests/t/cds/multi/ReadMe"
table2 = ascii.read(data_remote_multi1, readme=readme_remote_multi)
table3 = ascii.read(data_remote_multi2, readme=readme_remote_multi)

# Open remote single file
table4 = ascii.read("https://raw.githubusercontent.com/nudomarinero/astropy/master/astropy/io/ascii/tests/t/cds.dat")

# Open remote Vizier tables
data_remote_vizier = "https://raw.githubusercontent.com/nudomarinero/astropy/master/astropy/io/ascii/tests/t/vizier/table1.dat"
readme_remote_vizier = "https://raw.githubusercontent.com/nudomarinero/astropy/master/astropy/io/ascii/tests/t/vizier/ReadMe"
table5 = ascii.read(data_remote_vizier, readme=readme_remote_vizier)
```

Would it be worth to write tests for these cases?