The parsing error of the multitoken argument with negative numbers

The parsing error of the multitoken argument with negative numbers occurs while setting negative numbers after the first number.

For example, if we set an option
```cpp
("point,p", po::value>(&options.point)->multitoken(), "The point")
```
and enter in the command line the following values
```
prog -p -1 2 3
```
It works correctly.
However, if we enter the values given bellow
```
prog -p -1 -2 3
```
the parsing error occurs
```
unrecognised option '-2'
```

This error exists in Boost 1.65.1 as well.

Reproduce the issue with the shown multitoken option and both command-line examples, then trace the command-line option parsing path that classifies negative values after the first token. Done means the `-p -1 -2 3` invocation is accepted and produces the same vector as `-p -1 2 3`, with coverage for the reported regression.

Scritto dal modello di indicizzazione a partire dal testo della issue.