add compile_string() for compiling strings to css

[hhaensel]

This PR fixes #2.

• modify sass_make_data_context() to always copy a Julia AbstractString to sass in order to prevent segfaults
• add compile_string() with corrected sass_make_data_context()
• add respective tests

The following code example from sass helped me to develop this PR
https://github.com/sass/libsass/blob/master/docs/api-context-example.md

Not sure whether this API is the optimal solution. One could think of having only compile() with multiple dispatch, or defining compile_sass(), compile_scss() with the respective setting of is_indented_syntax_src.

compile_sass(data; kwargs...) = compile_string(data; compile_sass = true, kwargs...)
compile_scss(data; kwargs...) = compile_string(data; compile_sass = false, kwargs...)

@piever Happy to hear what you think.

[piever]

Thanks for the PR! In terms of API, the second approach makes the most to me. There isn't really a custom type for filepaths in julia, so having

compile_file # infers format from extension
compile_sass # takes a string (or maybe even an `IO` object)
compile_scss # takes a string (or maybe even an `IO` object)

makes the most sense to me. compile_string can remain as an internal function to avoid code duplication.

[hhaensel]

Some more thoughts;

• we could have some automatic format detection of sass/scss in compile_string
• I have not included source_maps in compile_string, because I thought that this would be a rather strange use case to work with string css but source map files. If you think differently we should re-include those parts (not much of a deal).

[hhaensel]

@piever Shall we merge it as is or do you think that more modifications should be done?

[piever]

LGTM! Still, it could be nice to add a small example in the README, as otherwise one can only found out about Sass.compile_sass and Sass.compile_scss by reading the source code.

[codecov-commenter]

Codecov Report

Merging #26 (d680ac4) into master (64a2c2a) will increase coverage by 19.38%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master      #26       +/-   ##
===========================================
+ Coverage   21.79%   41.17%   +19.38%     
===========================================
  Files           6        6               
  Lines         156      187       +31     
===========================================
+ Hits           34       77       +43     
+ Misses        122      110       -12     

Impacted Files	Coverage Δ
src/context.jl	68.75% <100.00%> (+36.31%)	⬆️
src/julian_api.jl	78.26% <100.00%> (+15.29%)	⬆️

... and 1 file with indirect coverage changes