add: truthy for bool evaluation

[legendof-selda]

As described in #12, Sentinels truthyness depends on the context of the sentinel.
This allows us to set what the sentinel can mean, and by default sentinels are ambiguous and forces the user to give it a meaning.

Example

from sentinels import Sentinel

EOF = Sentinel('EOF')

# This will return a `ValueError` since EOF doesn't have a Boolean representation
# bool(EOF)
# However, we can compare a value with EOF and that does have a Boolean representation
line= read_line(file)
print(line is EOF)

Undefined = Sentinel('undefined', truthy=False)

bool(Undefined)
# >> False

Success = Sentinel('success', truthy=True)

bool(Success )
# >> True

[legendof-selda]

I haven't updated the documents yet. If this get's approved, I will update it then

[legendof-selda]

any update on this @taleinat ?