The command px has no default values

[oddcoder]

In radare2, px shows x opcodes after the command. In rair, px need to have a size specified to work. To look like more radare2 we may want to use the same default value.

By @gogo2464

Moved from rair repor issue https://github.com/Rair-Project/rair/issues/12

[oddcoder]

An idea on how to do that would be implementing an rair-env var like printHex.defaultSize where it will hold the default side of input instead of having hardcoded default.