Export `RE.Regex`, and make constructors easier.

[panxiaoguang]

I want to have a regex search search for biosequences, and it can be changed by a value.

for example,

start="AGCAT"
re=@biore_str(string(start,"[AGCT]+"),"d")

it always repored an error, but I want it can be changed by start value?

[jakobnissen]

You can do this by explicitly constructing the regex:

julia> start="AGCAT"
"AGCAT"

julia> BioSequences.RE.Regex{DNA}(start * "[ACGT]+", :pcre)
biore"AGCAT[ACGT]+"dna

This functionality ought to be exposed - in general it's good style that, whenever you have a macro exported, also export the equivalent function.

[jakobnissen]

Resolved in #169