[SUGGESTION] execute commands directly rather than through a shell

[ghost]

We can execute the commands directly rather than through another shell to avoid unnecessary nesting of shells.

By replacing

#define BLOCK(cmd, interval, signal) \
    { "echo \"$(" cmd ")\"", interval, signal }

with

#define BLOCK(cmd, interval, signal) \
    { cmd, interval, signal }

and

execl("/bin/sh", "sh", "-c", blocks[i].command, (char*)NULL);

with

execlp(blocks[i].command, (char*)NULL);

we can execute each block without the need of an extra shell.

I apologise for not having sent a PR, I have been having issues with Github recently.

[UtkarshVerma]

Running the command in another shell is one unsatisfying workaround which I've had to live to with. It is discussed in more detail in #24. Therefore, I will be closing this in favour of that issue.