add config file support

[AASanchezA]

first implementation for config file support issue #17

[scw1109]

Parameter x need to parse by net_or_host_param() before checking.
Otherwise it will not handle the subnet and alias cases.

[AASanchezA]

please check 2f0b464

[scw1109]

Any idea what would the startup performance looks like when putting in a large configuration file?
I did i brief try with 10k more line of subnet configuration, it doesn't look good.
But I am not familiar with vpn-slice enough to get into details.

[AASanchezA]

for my use case I try with 10-20 lines, if you need 10k line are you sure you need to slice the vpn tunel? I will check the performance when I get some time.
thanks for the comments

[AASanchezA]

I tested the file parsing, here are the results:

• 10 lines file

got config file: vpn_slice_10.txt
elapsed time: 8.869171142578125e-05 s; number lines: 10
Must be called as vpnc-script, with $reason set; use --help for more information

• 10k lines

got config file: vpn_slice_10k.txt
elapsed time: 0.0041501522064208984 s; number lines: 10000
Must be called as vpnc-script, with $reason set; use --help for more information

the bottle neck should be somewhere else, not in the file parsing

[dlenski]

Is there a reason that you wrote this to accept only routes/hostnames/aliases in the configuration file, and not other arbitrary arguments?

[AASanchezA]

no reason, I took the idea from here #17 (comment)
but, yeah it could be extended to use a yml or json or toml, what do you think?

[AASanchezA]

last commit d988aa3 enable from_prefix_chars option for ArgumentParser which enable the parse from file by prefixing the character "@" to config filename

vpn-slice @list_of_routes.txt

with this change, option --config could be remove, what do you think?
also remove some complexity introduce by file parsing