plot_implicit: Explicitly specify x and y

See: https://groups.google.com/d/topic/sympy/isw07OhB6OI/discussion
simplify handling of x_var and y_var
allow arbitrary axis names
label plot axes and raise error if all expr not univariate
edit plot docstrings
label y-axis as 'f(x)' instead of 'arbitrary'
Merge pull request #7419 from amitsaha/plot_implicit

plot_implicit: Explicitly specify x and y