Add emulated 'clear screen' linux terminal capability for windows cmd

[tkhurana96]

This commit includes(for now) only the emulated version of clear screen terminal capability of linux terminals.

• [x] Add appveyor
• [x] Add code coverage
• [x] Merge Win+Linux code
• [x] Find way to turn off exceptions
• [x] Delete extraneous functions
• [ ] Expand tests for non-exception
• [x] Update docs examples
• [x] Configure doc examples builder
• [x] Return proxy object from str cap api
• [x] Add os.flush (i.e. flush the stream) wherever necessary
• [x] Write to stream without using the properties set on the stream os

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (develop@f0dbf3b). Click here to learn what that means. The diff coverage is 67.18%.

@@            Coverage Diff             @@
##             develop       #6   +/-   ##
==========================================
  Coverage           ?   69.56%           
==========================================
  Files              ?        1           
  Lines              ?      161           
  Branches           ?        0           
==========================================
  Hits               ?      112           
  Misses             ?       49           
  Partials           ?        0

Impacted Files	Coverage Δ
include/termdb.hpp	69.56% <67.18%> (ø)

[agauniyal]

@tkhurana96

#include <iostream>
using namespace std;

enum class Except{ OFF, ON };

template<Except E>
class Base {
public:
   Base();
};

template<> Base<Except::ON>::Base() { cout << "ONNN\n"; }
template<> Base<Except::OFF>::Base() { cout << "OFFF\n"; }

template<Except E=Except::ON>
class A : public Base<E> {
public:
    void say() { std::cout << "Hello\n";}
};

int main() {
    A a{};
    a.say();

    A<Except::OFF> b{};
    b.say();
}