allow em-values

[danielgruber]

filter: blur(0.25em); doesn't work
filter: blur(4px); works

[Schepp]

Although you are technically right, I don't see that much incentive to fix this bug. What do you think? Is it that much of a problem?

[iamvdo]

@danielgruber: I personnally add this unit (and also % and rem) when adapting this module for pleeease-filters (with no options for now, that means 1em fallbacks to 16px). You can test it out here: http://pleeease.io/playground.html?div%20%7B%0A%20%20filter:%20blur(.25em)%0A%7D