Bug: progress=true consumes the iterator

[perrygeo]

Describe the bug
The progress bar feature (introducing in #300 and release in 0.20) consumes the iterator in order to get the length of the sequence. This leaves zero features left to analyze; all zonal_stats(..., progress=True) calls will return []

To Reproduce

Steps to reproduce the behavior:
Install rasterstats >= 0.20 and run

from rasterstats import zonal_stats

v = "tests/data/polygons.shp"
r = "tests/data/slope.tif"

a = zonal_stats(v, r, progress=False)
b = zonal_stats(v, r, progress=True)
assert a == b   # FAILS

Potential Solutions

The design of the io module is very explicitly a stateful generator (hence the gen in gen_zonal_stats) so we can't consume it to determine the length.

1. Remove the explicit length (cons: progress bar will be useless with a sequence of unknown length)
2. Iterate twice (con: performance hit but at least it would be correct)
3. Add a length-hinting private function to the io module so that we can determine the length of any vector input efficiently. (con: may not be possible to do determine length efficiently for all inputs)

None of these are good options. If we can't figure out a solution, I'll need to remove the progress bar for correctness sake.

[abovzv]

@perrygeo I made a fix in the pr #304 , it could be better to pass a GeoDataframe instead of str