Improve target rules support

[Xewar313]

Target rules are rules with default_target set on true, or the first rule in the file.
New functionality that could be supported by the target rules:

• Mark wildcards as not allowed for target rules (target rules do not allow to use wildcards, to avoid ambiguity during execution)
• Don't mark log sections in target rules, since this behavior is supported (fix: 2142 log directive in default target rule snakemake/snakemake#2191)