Add JSHint to project, and fix errors.

[Belema]

Added JSHint to project and to build.

Fixed all 18 errors below,

slug.js: line 5, col 19, Expected '{' and instead saw 'return'.
slug.js: line 8, col 60, Missing semicolon.
slug.js: line 15, col 9, Expected '{' and instead saw 'opts'.
slug.js: line 24, col 9, Expected '{' and instead saw 'opts'.
slug.js: line 27, col 18, 'key' is already defined.
slug.js: line 29, col 13, Expected '{' and instead saw 'continue'.
slug.js: line 33, col 13, Expected '{' and instead saw 'lengths'.
slug.js: line 37, col 22, 'i' is already defined.
slug.js: line 37, col 29, 'l' is already defined.
slug.js: line 44, col 20, Expected '{' and instead saw 'return'.
slug.js: line 45, col 10, Don't make functions within a loop.
slug.js: line 61, col 26, Expected '{' and instead saw 'char'.
slug.js: line 68, col 7, Expected '{' and instead saw 'result'.
slug.js: line 70, col 2, Unnecessary semicolon.
slug.js: line 193, col 13, Expected '{' and instead saw 'continue'.
slug.js: line 197, col 40, Missing semicolon.
slug.js: line 203, col 18, 'key' is already defined.
slug.js: line 205, col 13, Expected '{' and instead saw 'continue'.

Note that,

• I have used [].forEach() where appropriate, because it is as well supported in browsers as [].some() is, so this is a non breaking change.
• I haven't used Object.keys() because it is not as well supported as [].some, so some browsers would have suffered from the change.
• I have not changed the algorithm at all, as this in not part of this work.

[lillem4n]

I'm trying to sum up all PR:s from this project to a fork here https://github.com/larvit/larvitslugify until this project comes alive again. Could you pretty please add this PR to that fork?