Also fixes the incompatibility between store_into and scan and action:
when the three methods above were called, being all based on the
(unique) action, the last one would overwrite the previous ones.
This issue was making the parser strictly dependant on the order
of the scan/store_into/action calls making them mutually exclusive.
It is possible to bind arguments to a variable storing their value, as an
alternative to explicitly calling ``program.get<T>(arg_name)`` or ``program[arg_name]``
This is currently implementeted for variables of type ``bool`` (this also
implicitly calls ``flag()``), ``int``, ``double``, ``std::string`` and
``std::vector<std::string>``. If the argument is not specified in the command
line, the default value (if set) is set into the variable.
```cpp
bool flagvar = false;
program.add_argument("--flagvar").store_into(flagvar);
int intvar = 0;
program.add_argument("--intvar").store_into(intvar);
double doublevar = 0;
program.add_argument("--doublevar").store_into(doublevar);
std::string strvar;
program.add_argument("--strvar").store_into(strvar);
std::vector<std::string> strvar_repeated;
program.add_argument("--strvar-repeated").append().store_into(strvar_repeated);
std::vector<std::string> strvar_multi_valued;
program.add_argument("--strvar-multi-valued").nargs(2).store_into(strvar_multi_valued);
```
