Add methods concerning the previous line

Add additional methods for inspecting the indentation level

Add additional methods to `*prompt.Prompt`

Change the signature of ExecuteOnEnterCallback

Merge pull request #15 from elk-language/feature/support_grapheme_clu…

…sters

Fix horizontal cursor movements when there are grapheme clusters

Reset formatting after rendering a styled token

Add more formatting options for tokens

Improve the lexer implementation

Document changes in the README