Compile regexp's at build time. Use raw string literals for patterns …

[mrngm]

…to avoid escaping

This avoids patterns like "\\.id_rsa.*$", where the \ needs to be escaped such that Go's string interpreter correctly feeds \.id_rsa.*$ to regexp.MustCompile (or regexp.Compile).

This improves legibility.

Note that in RE2, \n is still interpreted as a newline by regexp, whereas prior to this PR, Go's string interpreter would have converted it to the newline character.

There is also a minor optimization in func NewIgnores, that reuses the group variable inside the loop, by definining it outside of the for.

In theory, these changes will impact #152 positively.