Implement @dec_str

Fixes #26:
```julia
julia> dec"10_000.000_000_1"
10000.0000001
```

README.md

@@ -56,7 +56,15 @@ julia> parse(Decimal, x)
 julia> tryparse(Decimal, x)
 0.2
 ```
-Parsing support scientific notation.
+Parsing support scientific notation.  Alternatively, you can use the `@dec_str`
+macro, which also supports the thousands separator `_`:
+```julia
+julia> dec"0.2"
+0.2
+
+julia> dec"1_000.000_001"
+1000.000001
+```
 
 ### Conversion
 

src/Decimals.jl

@@ -6,7 +6,8 @@ module Decimals
 
 export Decimal,
        number,
-       normalize
+       normalize,
+       @dec_str
 
 const DIGITS = 20
 

src/parse.jl

@@ -1,3 +1,31 @@
+macro dec_str(s)
+    # Taken from @big_str in Base
+    msg = "Invalid decimal: $s"
+    throw_error = :(throw(ArgumentError($msg)))
+
+    if '_' in s
+        # remove _ in s[2:end-1]
+        bf = IOBuffer(maxsize=lastindex(s))
+        c = s[1]
+        print(bf, c)
+        is_prev_underscore = (c == '_')
+        is_prev_dot = (c == '.')
+        for c in SubString(s, 2, lastindex(s) - 1)
+            c != '_' && print(bf, c)
+            c == '_' && is_prev_dot && return throw_error
+            c == '.' && is_prev_underscore && return throw_error
+            is_prev_underscore = (c == '_')
+            is_prev_dot = (c == '.')
+        end
+        print(bf, s[end])
+        s = String(take!(bf))
+    end
+
+    x = tryparse(Decimal, s)
+    x === nothing || return x
+    return throw_error
+end
+
 function Base.tryparse(::Type{Decimal}, str::AbstractString)
     regex = Regex(string(
         "^",
@@ -44,4 +72,3 @@ function Base.parse(::Type{Decimal}, str::AbstractString)
     isnothing(x) && throw(ArgumentError("Invalid decimal: $str"))
     return x
 end
-

test/test_parse.jl

@@ -27,4 +27,16 @@
         @test isnothing(tryparse(Decimal, "1e1e1"))
         @test isnothing(tryparse(Decimal, "1-1"))
     end
+
+    @testset "@dec_str" begin
+        @test dec"1.123" == Decimal(0, 1123, -3)
+        @test dec"-1.123" == Decimal(1, 1123, -3)
+        @test dec"123" == Decimal(0, 123, 0)
+        @test dec"-123" == Decimal(1, 123, 0)
+        @test dec"1_000.002" == Decimal(0, 1000002, -3)
+        @test dec"1_000_000.002" == Decimal(0, 1000000002, -3)
+
+        @test_throws ArgumentError dec"100_"
+        @test_throws ArgumentError dec"100_.0"
+    end
 end