add retries request

[emilevauge]

This PR adds retry request in case of network error.
You can enable retry in your global config file adding [retry].
The default config set a maximum storage in memory by request of 2 Mo and a maximum attempts of (number servers in backend) -1.
Your can override this:

# Enable retry sending request if network error
#
# Optional
#
[retry]

# Number of attempts
#
# Optional
# Default: (number servers in backend) -1
#
attempts = 3

# Sets the maximum request body to be stored in memory in Mo
#
# Optional
# Default: 2
#
maxMem = 3

Fixes #210

[vdemeester]

@emilevauge this is general to all providers, you cannot set different values by providers. Is this intended ? 👼

[emilevauge]

If you think I should add the possibility to override by provider, I'm OK to add it ;)