Added support for seekg and tellg functions

[TAR-ALEX]

These code change include added support for seekg and tellg functions. These functions can be useful for determining the size of the uncompressed output. For my particular use case I need to store a few positions in a file and come back to them later on a second pass. The new seek function is capable of seeking forward. If a backwards seek is triggered and it is already buffered it will just jump to that position, but when the position is not in the buffer it will reset the stream and seek from the zero position. A relative seek from the end of the stream is not possible and will throw an exception.

Here is the code that I used for testing (I also changed the buffer size to 100):

#include <iostream>
#include <fstream>
#include <bxzstr.hpp>

using namespace std;

int main(){
    bxz::ifstream in("./src.tar.gz");
    char c;
    in.seekg(2460);
    cout << in.tellg() << endl;
    in.seekg(2458);
    cout << in.tellg() << endl;
    int pos = 0;
    while(in.get(c)){
        if(c == '#' && pos == 0) pos = in.tellg()-1; 
        cout << in.tellg() << endl;
    }
    cout << endl;

    in.clear();
    in.seekg(pos);
    while(in.get(c)){
        cout << c;
    }
    cout << endl;
    
    return 0;
}

Here is the file I used as well:
src.tar.gz

[tmaklin]

This is great, thanks a lot @TAR-ALEX! I'll merge it right away.