Are enhancers and silencers considered as epigenetic modifications?
No. 

Epigenetic information is (by definition) NOT included in the nucleotide sequence, but affects genetic expression.

Enhancers/silencers are themselves nucleotide sequences, and therefore not epigenetic information.

Methylation is an example of epigenetic information, but a DNA sequence itself is genetic information, even if it affects a gene.

CgATgCCAg= Genetic

Acetylation of a histone/DNA methylation= epigenetic

Hope that helps! 

Further reading: http://www.whatisepigenetics.com/fundamentals/